Enhanced hybrid systems and methods for characterizing stress in chemically strengthened transparent substrates

ABSTRACT

The hybrid measurement system includes an evanescent prism coupling spectroscopy (EPCS) sub-system and a light-scattering polarimetry (LSP) sub-system. The EPCS sub-system includes an EPCS light source system optically coupled to an EPCS detector system through an EPCS coupling prism. The LSP sub-system includes an LSP light source optically coupled to an optical compensator, which in turn is optically coupled to a LSP detector system via a LSP coupling prism. A support structure supports the EPCS and LSP coupling prisms to define a coupling prism assembly, which supports the two prisms at a measurement location. Stress measurements made using the EPCS and LSP sub-systems are combined to fully characterize the stress properties of a transparent chemically strengthened substrate. Methods of processing the EPCS and LSP measurements and enhanced configurations of the EPCS and LPS sub-systems to improve measurement accuracy are also disclosed.

This application claims the benefit of priority of U.S. Provisional Application Ser. No. 63/152,021 filed on Feb. 22, 2021 the content of which is relied upon and incorporated herein by reference in its entirety.

FIELD

The present disclosure relates to characterizing stress in transparent chemically strengthened substrates, and in particular relates to enhanced hybrid systems and methods for characterizing stress chemically strengthened transparent substrates.

BACKGROUND

Transparent substrates that have undergone a chemical strengthening process exhibit increased resistance to scratching and breaking. Such substrates extremely useful for a variety of display applications ranging from television screens to computer screens to mobile hand-held device screens to watches. An example chemical strengthening process is an ion-exchange (IOX) process, whereby ions in a near-surface region of a glass-based substrate are exchanged for external ions, e.g., from a salt bath.

Manufacturing transparent chemically strengthened (CS) substrates requires characterizing their stress characteristics to ensure the CS substrates have the desired levels of chemical strengthening suitable for the given application. The characterization typically requires measuring a stress profile of the CS substrate from the surface to the center, along with related stress parameters, such as the surface compressive stress, the knee stress, the spike depth of layer, the total depth of layer, the depth of compression and the central tension. Other stress-related parameters include the variation in the birefringence with depth into the CS substrate.

There are two main methods used to characterize stress of a transparent CS substrate. The first utilizes evanescent prism coupling spectroscopy (EPCS). The EPCS method using a coupling prism to couple light into guided modes supported by a near-surface waveguide (NSWG) formed in the substrate, e.g., by an IOX process. A coupling prism is also used to couple light out of the NSWG to form a guided mode spectrum. The guide mode spectrum includes a transverse electric (TE) mode spectrum with TE mode lines and a transverse magnetic (TM) spectrum with TM mode lines. The TE and TM modes lines are analyzed to extract stress-related characteristics, including a stress profile. The EPCS method is particularly useful for characterizing stress in the near-surface region of the CS substrate (e.g., surface compressive stress and spike depth of layer) but is not useful for characterizing a central tension CT and depth of compression DOC that reside deeper within the substrate.

The second main method utilizes light-scattering polarimetry (LSP). In LSP, the CS substrate is irradiated with input laser light at a relatively shallow angle through a coupling prism. The laser light polarization is varied continuously between different polarization states using an optical compensator. The scattered light is detected by an image sensor. Stress in the CS substrate causes optical retardation along the light path, with the amount of stress being proportional to the derivative of the optical retardation. The amount of optical retardation can be determined from the detected scattered light intensity distribution, which varies due to the constructive and destructive interference for the different effective path lengths of the detected light. The LSP method is useful for measuring certain stress-related properties, such as the central tension (CT) and depth of compression (DOC) but is not useful for measuring near-surface stress-related properties.

Presently, to fully characterize the stress profile of a CS substrate from the surface to the center, the CS substrate is first measured using a EPCS measurement system and is them moved to a LSP measurement system and the two measurement stitched together. This is time consuming and introduces the risk of breakage by having to handle the CS substrate when moving the CS substrate between the two measurement systems.

It would therefore be more advantageous to have a single measurement system capable of performing enhanced EPCS and LSP measurements.

SUMMARY

The hybrid measurement systems and methods disclosed herein enable a full stress characterization of a transparent CS substrate, including the surface stress S(0), the near-surface compressive stress profile S(x) including the knee stress S_(k)=S(x_(k)), the depth of layer DOL, the central tension CT, and the depth of compression DOC. The full stress characterization is obtained by combining the stress calculations using both EPCS and LSP measurements.

An embodiment of the disclosure is directed to a system for characterizing stress in a CS substrate having a top surface and a near-surface waveguide. The system comprises: an EPCS sub-system comprising a EPCS light source system and a EPCS detector system in optical communication through an EPCS coupling prism having a EPCS coupling surface; a LSP sub-system comprising a LSP light source system, an optical compensator and a LSP detector system in optical communication with the optical compensator through an LSP coupling prism having a LSP coupling surface; a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces resides substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane. In an example, the EPC sub-system and/or the LSP sub-system have the enhanced configurations as described below.

Another embodiment of the disclosure is directed to a method of measuring first and second stress characteristics of a CS substrate having a surface and a near-surface waveguide, comprising: operably disposing the surface of the CS substrate relative to a coupling prism assembly at a measurement location, the coupling prism assembly comprising an EPCS coupling prism and a LSP coupling prism to respectively define adjacent EPCS and LSP coupling interfaces; performing a EPCS measurement of the CS substrate using the EPCS coupling interface to obtain the first stress characteristics and performing a LSP measurement of the CS substrate using the LSP coupling interface to obtain the second stress characteristics without removing either the coupling prism assembly or the CS substrate from the measurement location; and combining the first and second stress characteristics to establish a full stress characterization of the CS substrate. In an example, the EPC sub-system and/or the LSP sub-systems having the enhanced configurations as described below can be used to carry out enhanced methods of measuring the first and second stress characteristics.

Another embodiment of the disclosure is directed to an EPCS system for characterizing stress in a CS substrate having a surface and a near-surface waveguide, comprising:

-   -   a) an EPCS light source system comprising:         -   i) an EPCS light source that emits a multiwavelength EPCS             light beam;         -   ii) an optical filter assembly configured to sequentially             filter the multiwavelength EPCS light beam to form a             sequence of filtered EPCS light beams having different             wavelengths;         -   iii) a light guide assembly that transfers the sequence of             filtered EPCS light beams as guided light to a focusing             optical system arranged to receive the transferred filtered             EPCS light beams and form therefrom a sequence of filtered             and focused EPCS light beams;     -   b) an EPCS coupling prism that forms an EPCS coupling surface         with the surface of the CS substrate and that receives and         couples the sequence of filtered and focused EPCS light beams         into and out of the near-surface waveguide at the EPCS coupling         surface to form a sequence of filtered and reflected EPCS light         beams that respectively comprise mode spectra of the         near-surface waveguide for the corresponding filtered and         reflected EPCS light beam; and     -   c) an EPCS detector system comprising:         -   i) a switchable polarization filter operably connected to a             polarization controller to sequentially perform transverse             magnetic (TM) and transverse electric (TE) polarization             filtering of the sequence of filtered and reflected EPCS             light beams to form TM and TE filtered and reflected EPCS             light beams respectively comprising TM and TE mode spectra             of the near-surface waveguide; and         -   ii) an EPCS digital detector configured to sequentially             detect the sequence of TM and TE filtered and reflected EPCS             light beams to sequentially capture TM and TE images of the             respective TM and TE mode spectra of the near-surface             waveguide at the different filter wavelengths.

Another embodiment of the disclosure is directed to the EPCS system described above and herein, wherein the optical filter assembly comprises an optical filter wheel.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the optical filter wheel comprises a plurality of optical filter assemblies each comprising an optical filter and a correcting member configured to provide focus correction at the given filter wavelength.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the EPCS light source comprises at least one broadband light source element.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the EPCS light source comprises multiple light source elements that respectively emit either simultaneously or sequentially different EPCS light beams respectively having different wavelengths.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the EPCS light beams having different wavelengths are combined to travel along a common axis using one or more light-selective elements to form the multiwavelength EPCS light beam.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the one or more light-selective elements comprises one or more dichroic mirrors.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, further comprising multiple correction lenses respectively operably disposed relative to the multiple light source elements to facilitate optical coupling of the light beams into the input end of the light guide.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, further comprising a light diffuser arranged adjacent the input end of the light guide.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the light guide is liquid filled.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the CS substrate comprises a glass material, a glass-ceramic material or a crystalline material, and wherein the near-surface waveguide of the CS substrate is defined by a near-surface spike region and a deep region.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the light guide assembly comprises a light guide with an output end, the EPCS detector system comprises an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil by the focusing optical system.

Another embodiment of the disclosure is directed to a hybrid system for characterizing stress in a CS substrate having a top surface and a near-surface waveguide, comprises the EPCS system as described above and disclosed herein: a scattered light polarimetry (LSP) sub-system comprising a LSP light source system, an optical compensator and a LSP detector system in optical communication with the optical compensator through an LSP coupling prism having a LSP coupling surface; a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces reside substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, for characterizing stress in a CS substrate having a surface and a near-surface waveguide, comprising:

-   -   a) an EPCS light source system comprising:         -   i) an EPCS light source that emits a multiwavelength EPCS             light beam;         -   ii) an optical filter assembly configured to sequentially             filter the multiwavelength EPCS light beam to form a             sequence of filtered EPCS light beams having different             filter wavelengths;         -   iii) a light guide assembly configured to transfer the             sequence of filtered EPCS light beams as guided light to a             focusing optical system arranged to receive the transferred             filtered EPCS light beams and form therefrom a sequence of             filtered and focused EPCS light beams;     -   b) an EPCS coupling prism that forms a EPCS coupling surface         with the surface of the CS substrate and that receives and         couples the focused sequentially filtered EPCS light beam out of         the near-surface waveguide at the EPCS coupling surface to form         a reflected and sequentially filtered EPCS light beam that         comprises at least first and second mode spectra of the         near-surface waveguide for the at least first and second filter         wavelengths; and     -   c) an EPCS detector system comprising:         -   i) at least one switchable polarization filter configured to             sequentially perform transverse magnetic (TM) and transverse             electric (TE) polarization filtering of the reflected and             sequentially filtered EPCS light beam to form at least first             and second TM and TE reflected and sequentially filtered             EPCS light beams respectively comprising first and second TM             and TE mode spectra of the near-surface waveguide at the at             least first and second wavelengths, respectively; and         -   ii) at least first and second EPCS digital detectors             configured to respectively detect the at least first and             second TM and TE reflected and sequentially filtered EPCS             light beams to capture respective at least first and second             TM and TE images of the first and TM and TE mode spectra of             the near-surface waveguide.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the at least first and second EPCS digital detectors reside along respective at least first and second detector axes, and wherein the at least one switchable polarization filter comprises at least first and second switchable polarization filters respectively arranged along the at least first and second detector axes and upstream of the corresponding one of the at least first and second EPCS digital detectors.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the CS substrate comprises a glass material, a glass-ceramic material or a crystalline material, and wherein the near-surface waveguide of the CS substrate is defined by a near-surface spike region and a deep region.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the light guide assembly comprises a light guide with an output end, the EPCS detector system comprises an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil.

Another embodiment of the disclosure is directed to a system for characterizing stress in a CS substrate having a top surface and a near-surface waveguide, comprising: The EPCS system described above and disclosed herein; a scattered light polarimetry (LSP) sub-system comprising a LSP light source system, an optical compensator and a LSP detector system in optical communication with the optical compensator through an LSP coupling prism having a LSP coupling surface; a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces resides substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, for characterizing stress in a CS substrate having a surface and a near-surface waveguide, comprising:

-   -   a) an EPCS light source system comprising:         -   i) an EPCS light source that emits a multiwavelength EPCS             light beam comprising multiple wavelengths;         -   ii) a light guide assembly that transfers the             multiwavelength EPCS light beam from the EPCS light source             to a focusing optical system arranged to receive the             multiwavelength EPCS light beam and form a focused             multiwavelength EPCS light beam;     -   b) an EPCS coupling prism that forms an EPCS coupling surface         with the surface of the CS substrate and that receives and         couples the multiwavelength EPCS light beam into and out of the         near-surface waveguide at the EPCS coupling surface to form a         reflected multiwavelength EPCS light beam that comprises mode         spectra of the near-surface waveguide for the corresponding         multiple wavelengths of the multiwavelength EPCS light beam; and     -   c) an EPCS detector system comprising:         -   i) a switchable polarization filter operably disposed to             receive the reflected multiwavelength EPCS light beam and             sequentially form a transverse magnetic polarized (TM)             multiwavelength EPCS light beam and a transverse electric             polarized (TE) multiwavelength EPCS light beam.         -   ii) an optical filter assembly operably disposed to             sequentially filter the TM and TE multiwavelength EPCS light             beams at two or more filter wavelengths to form two or more             sequentially filtered TM and TE EPCS light beams             respectively comprising the TM and TE mode spectra of the             two or more filter wavelengths;         -   iii) an EPCS digital detector configured to sequentially             detect the sequentially filtered TM and TE EPCS light beams             to sequentially capture TM and TE images of the respective             TM and TE mode spectra of the near-surface waveguide at the             two or more filter wavelengths.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the CS substrate comprises a glass material, a glass-ceramic material or a crystalline material, and wherein the near-surface waveguide of the CS substrate is defined by a near-surface spike region and a deep region.

Another embodiment of the disclosure is directed to the EPCS system as described above and disclosed herein, wherein the light guide assembly comprises a light guide with an output end, the EPCS detector system comprises an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil by the focusing optical system.

Another embodiment of the disclosure is directed to A system for characterizing stress in a CS substrate having a top surface and a near-surface waveguide, comprising: The EPCS system as described above and disclosed herein; a scattered light polarimetry (LSP) sub-system comprising a LSP light source system, an optical compensator and a LSP detector system in optical communication with the optical compensator through an LSP coupling prism having a LSP coupling surface; a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces resides substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane.

Another embodiment of the disclosure is directed to a method of performing evanescent prism coupling spectroscopy for characterizing stress in a CS substrate having a surface and a near-surface waveguide, comprising:

-   -   a) forming a multiwavelength EPCS light beam having multiple         wavelengths;     -   b) sequentially filtering the EPCS multiwavelength light beam to         form a sequence of filtered EPCS light beams each having a         different one of the multiple wavelengths;     -   c) transferring the sequence of EPCS filtered light beams         through a light guide to a focusing optical system to form a         focused sequence of EPCS filtered light beams;     -   d) directing the focused sequence of filtered EPCS light beams         to an EPCS coupling prism that forms an EPCS coupling surface         with the surface of the CS substrate and that receives and         couples the sequence of filtered EPCS light beams into and out         of the near-surface waveguide at the EPCS coupling surface to         form a sequence of reflected and filtered EPCS light beams that         respectively comprise mode spectra of the near-surface waveguide         at one of the multiple wavelengths;     -   e) sequentially polarizing each of the reflected and filtered         EPCS light beams to form for each filtered and reflected EPCS         light beam a transverse magnetic (TM) filtered and reflected         EPCS light beam and a transverse electric (TE) filtered and         reflected EPCS light beam; and     -   f) sequentially digitally detecting the TM and TE filtered and         reflected EPCS light beams to sequentially capture TM and TE         images of the respective TM and TE mode spectra of the         near-surface waveguide for the different multiple wavelengths.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein and further comprising processing the sequentially captured TM and TE images of the respective TM and TE mode spectra to characterize at least one stress-related property of the CS substrate.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein the sequential filtering comprises sequentially passing the multiwavelength light beam through optical filter assemblies supported by a filter wheel.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein each optical filter assembly comprises an optical filter and a correcting member configured to correct for wavelength-dependent focus errors.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein the sequential polarizing comprises passing the reflected and filtered EPCS light beams through a magnetically charged polarizing crystal operably connected to a polarization controller.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein the CS substrate comprises a glass material, a glass-ceramic material or a crystalline material, and wherein the near-surface waveguide of the CS substrate is defined by a near-surface spike region and a deep region.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein sequentially digitally detecting the TM and TE filtered and reflected EPCS light beams is performed using a single EPCS digital detector.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein sequentially digitally detecting the TM and TE filtered and reflected EPCS light beams is performed using multiple EPCS digital detectors that are spatially separated using dichroic mirrors.

Another embodiment of the disclosure is directed to the method as described above and disclosed herein, wherein the light guide has an output end, the sequence of reflected filtered EPCS light beams is received and processed by an EPCS detector system that has an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil by the focusing optical system.

Another embodiment of the disclosure is directed to a light scattering polarimetry (LSP) system for characterizing stress in a CS substrate having a body, a surface and a near-surface waveguide formed within the body, comprising:

-   -   a) a LSP light source system comprising in order along a first         system axis:         -   i) a laser diode that emits a LSP light beam having at least             1 microwatt of power and centered on a wavelength of 405             nanometers;         -   ii) a shutter system arranged to periodically block the LSP             light beam;         -   iii) a rotatable half-wave plate;         -   iv) a first fixed polarizer;         -   v) a first focusing lens;         -   vi) a light diffuser;         -   vii) a second focusing lens     -   b) an optical compensator arranged downstream of the LPS light         source and configured to impart to the LSP light beam a         time-varying polarization, the optical compensator comprising in         order along the system axis:         -   i) a polarizing beam splitter arranged to receive the LSP             light beam from the LSP light source and transmit a first             portion of the LSP light beam along the first system axis             and to direct a second portion of the LSP light beam along a             spectrometer axis;         -   ii) a spectrometer arranged along the spectrometer axis and             arranged to receive and spectrally process the second             portion of the LSP light beam;         -   iii) a second fixed polarizer;         -   iv) a variable polarizer that imparts the time-varying             polarization to the LSP light beam to form a time-varying             polarized LSP light beam;     -   c) an axially movable focusing lens arranged downstream of the         optical compensator and configured to receive and focus the         time-varying polarized LSP light beam form a focused         time-varying polarized LSP light beam;     -   d) an LSP coupling prism interfaced with surface of the CS         substrate to form a LSP coupling interface, wherein the focused         time-varying polarized LSP light beam is focused at the LSP         coupling interface to generate scattered light from         stress-induced features within the body of the CS substrate;     -   e) a LSP detector system arranged downstream of the LSP coupling         prism and arranged to receive the scattered light, the LSP         detector system comprising:         -   i) a LSP digital detector; and         -   ii) a collection optical system that collects and directs             the scattered light to the LSP digital detector to form an             LSP image at the digital detector.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein the variable polarizer comprises a liquid crystal variable retarder (LCVR) operably connected to a polarization controller.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein the LCVR is in operable communication with a temperature controller that maintains the LCVR within a select temperature range.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein the select temperature range is between 35° C. and 40° C.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein the axial movable focusing lens comprises a lens element supported by a lens support and wherein the lens support is mechanically attached to a linear motor.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein LSP light beam has at least 10 microwatts of power.

Another embodiment of the disclosure is directed to the LSP system as described above and as disclosed herein, wherein LSP light beam has at least 50 microwatts of power.

Another embodiment of the disclosure is directed to a system for characterizing stress in a CS substrate having a top surface and a near-surface waveguide, comprising: The LSP as described above and as disclosed herein; an evanescent prism coupling spectroscopy (EPCS) sub-system comprising a EPCS light source system and a EPCS detector system in optical communication through an EPCS coupling prism having a EPCS coupling surface; a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces resides substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane.

Another embodiment of the disclosure is a method of performing light scattering polarimetry (LSP) for characterizing stress in a CS substrate having a body, a surface and a near-surface waveguide that forms stress-related features within the body, comprising: generating a light beam from a laser diode having an output power of at least 1 microwatt and a center wavelength of 405 nm; directing a first portion of the light beam to a spectrometer to measure a wavelength of the light beam and an amount of power in the light beam; imparting to a second portion of the light beam a time-varying polarization using a temperature-controlled liquid crystal variable retarder (LCVR) to form a time-varying polarized light beam; focusing the time-varying polarized light beam onto a coupling surface formed by a coupling prism interfaced to the surface of the CS substrate to form scattered light from the stress-related features within the body of the CS substrate; and directing the scattered light to a digital detector to capture an LSP image at the digital detector.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, wherein the body of the CS substrate is made of a glass material.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, further comprising maintaining the LCVR at temperature in the range from 35° C. to 40° C.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, wherein the time-varying polarized light beam follows a beam path through the body of the CS substrate and further comprising estimating a beam center of the light beam by: performing a tilted Gaussian fit to the scattered light for select depths into the body of the CS substrate along the beam path to define a first set of beam centers; and fitting a first line through the first set of beam centers to define a first fitted line.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, further comprising estimating a center point of the time-varying polarized light beam by fitting a second line through a second set of beam centers to define a second fitted line, and identifying the center point where the first and second fitted lines cross.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, wherein the time-varying polarized light beam follows a beam path through the body of the CS substrate and further comprising determining an entrance point of the time-varying polarized light beam into the CS substrate by: Identifying an edge intensity profile of the LSP image along one of the first and second fitted lines where the edge intensity transitions from a maximum value I_(MAX) to a minimum value I_(MIN) representative of a background intensity value; and determining a half-maximum intensity value I_(1/2) midway between the maximum and minimum intensity values I_(MAX) and I_(MIN) and defining the entrance point to be at the half-maximum intensity value I_(1/2).

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, wherein the LSP light beam has at least 10 microwatts of power.

Another embodiment of the disclosure is directed to the method as described above and as disclosed herein, wherein the LSP light beam has at least 50 microwatts of power.

Additional features and advantages are set forth in the Detailed Description that follows, and in part will be apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which:

FIG. 1A is an elevated view of an example transparent CS substrate in the form of a planar sheet.

FIG. 1B is an example refractive index profile n(z) versus z of an example transparent CS substrate showing a near-surface spike region (R1), a deeper region (R2) and a bulk region (R3), with a knee (KN) at the transition between the regions R1 and R2.

FIG. 2A is a schematic diagram of the hybrid EPCS-LSP measurement system as disclosed herein for fully characterizing the stress of a transparent CS substrate.

FIG. 2B is a more detailed schematic diagram of the hybrid EPCS-LSP system of FIG. 2A, showing example configurations for the EPCS measurement sub-system and the LSP measurement sub-system.

FIG. 3A is a schematic diagram of an example EPCS sub-system of the hybrid EPCS-LSP system of FIG. 2A.

FIG. 3B is a schematic diagram of an example EPCS mode spectrum obtained by the EPCS sub-system, wherein the EPCS mode spectrum includes a TM mode spectrum with TM mode lines (fringes) and a TE mode spectrum with TE mode lines (fringes).

FIGS. 4A, 4B, and 4C are schematic diagrams of example LSP sub-systems of the hybrid EPCS-LSP system of FIG. 2A.

FIG. 4D is a close-up view of an LSP image formed on the digital detector of the LSP sub-system, wherein the LSP image includes two line images that form a cross or an “X” pattern, and wherein the LSP image and digital detector form a digital LSP image.

FIG. 5A is an elevated view of an example prism support structure for supporting the EPCS and LSP coupling prisms.

FIG. 5B is an elevated view of a cover plate mounted on the prism support structure of FIG. 5A.

FIGS. 6A and 6B are side views of the EPCS and LSP coupling prisms supported on a stable platform and illustrating an example method of forming a unitary molded prism support structure for the coupling prism assembly.

FIG. 6C is a side view of an example coupling prism assembly wherein the prism support structure is configured so that at least one of the EPCS and LSP coupling prisms is movable in one direction (e.g., the z-direction, as shown) relative to the other

FIG. 6D is a schematic diagram of an example hybrid EPCS-LSP measurement system wherein a single coupling prism is used for the EPCS sub-system and the LSP sub-system instead of two separate coupling prisms.

FIG. 7 is a cross-sectional view of an example prism support structure attached to an example support plenum of the hybrid system and showing an example movable substrate holder used to adjust measurement position on the CS substrate.

FIG. 8A is an elevated view of the support plenum showing the measurement aperture and pressure-vacuum (PV) bars of a vacuum system operably disposed within the measurement aperture to pneumatically engage the CS substrate to pull the CS substrate onto the coupling surfaces of the EPCS and LSP coupling prisms.

FIG. 8B is a close-up cross-sectional view of the support plenum and the measurement aperture showing an example configuration of the coupling prism assembly and the vacuum system.

FIG. 9 is a schematic representation of an example user interface as presented by the system controller, wherein the user interface includes a EPCS section that shows the EPCS mode spectrum and an LSP section that shows the LSP line images of the digital LSP image.

FIG. 10A is an example of an LSP section of the user interface showing an example digital LSP image and an intensity histogram of the digital LSP image.

FIG. 10B shows an example initial or raw digital LSP image along with a Gaussian-blurred (“blurred”) LSP image.

FIG. 10C shows an example threshold image as obtained by applying Ostu thresholding to the Gaussian-blurred image of FIG. 10B.

FIGS. 10D and 10E show an example of performing contour detection on an example Gaussian-blurred LSP image.

FIG. 11A is a close-up view of the CS substrate and the direction of the focused LSP light beam.

FIG. 11B is a close-up view of the edge portion of the CS substrate and showing the viewing angle of the LSP detector system relative to the focused LSP light beam.

FIG. 11C is similar to FIG. 11B and shows the scattered light beams that reach the LSP detector system and form the line images.

FIG. 11D shows another view of the LSP detector system and the CS substrate with the focused LSP light beam.

FIG. 11E is a schematic diagram showing dimensions and angles used to determine the CS substrate thickness.

FIG. 12A is a plot of the average computation time T in milliseconds (ms) needed to extract the phase φ of a noisy LSP signal versus the noise factor N for both the lock-in method (blue) and the sine method (green).

FIG. 12B is a plot of the absolute phase difference |Δφ| for versus the noise factor for the lock-in method (blue) and the sine method (green) for processing a noisy LSP signal.

FIGS. 13A and 13B are plots of the optical retardation OR (radians) versus the depth D (mm) into the CS substrate (“OR vs. D plots”), with FIG. 13A showing OR data collected by the LSP sub-system without using speckle reduction and FIG. 13 showing OR data collected by the LSP sub-system using speckle reduction.

FIGS. 14A and 14B are OR vs. D plots illustrating an example method of shifting the OR data to make the bend points BP1 and BP2 be symmetric around the mid-plane of the CS substrate.

FIG. 15A is an example OR vs. D plot that includes discrete data points (circles) and a fitted line to the OR vs. D data points, wherein the fitted line is formed using the “LinQuad” method disclosed herein.

FIG. 15B is a plot of the stress S (MPa) versus depth D (mm) based on the LinQuad fit to the OR vs. D data points of FIG. 15A.

FIG. 16A is an example OR vs. D plot that includes discrete data points (circles) and a fitted line to the OR vs. D data points, wherein the fitted line is formed using the power-spike method disclosed herein.

FIG. 16B is a plot of the stress S (MPa) versus depth D (mm) (“S vs. D plot”) based on the power-spike fit to the OR vs. D data points of FIG. 16A.

FIGS. 17A and 17B are OR vs. D plots that show LinQuad curve fits to the original (raw) OR vs. D data points (FIG. 17A) and to the OR vs. D data with the symmetric component removed (FIG. 17B).

FIGS. 18A and 18B are OR vs. D plots that illustrate using reduced area fitting regions when calculating a select stress parameter, with FIG. 18A showing reduced-area fitting regions at the bend points BP1 and BP2 to calculate the depth of compression DOC and FIG. 18B showing a reduced-area fitting region between the bend points BP1 and BP2 to calculate the central tension CT.

FIG. 19A is a OR vs. D plot and FIG. 19B is the corresponding S vs. D plot, wherein the curve fitting is done for the entire set of OR data.

FIG. 19C is a OR vs. D plot and FIG. 19D is the corresponding S vs. D plot, wherein the curve fitting is done for reduced set of OR data that excludes portions of the data near the opposite end points.

FIG. 20 is similar to FIG. 3A and illustrates an embodiment of the EPCS sub-system wherein the detector system includes an adjustable focusing lens, wherein the adjustability includes at least one of axial movement and changing the focal length.

FIGS. 21A and 21B are schematic illustrations of example support members use to form a focusing lens assembly for the EPCS sub-system to provide a means for adjusting the contrast of the captured mode spectrum.

FIG. 22A and FIG. 22B are similar to FIG. 3A and illustrate examples of an enhanced EPCS sub-system that employs a light guide assembly.

FIG. 22C is a close-up schematic diagram of an example configuration of the EPCS sub-system employing Kohler illumination with respect to the output end of the light guide and an entrance pupil of the EPCS detector system.

FIG. 23 is a schematic diagram of an example embodiment of the remote light source of the EPCS sub-system.

FIG. 24A is a front-on view of a filter wheel employed in the optical filter apparatus in the remote light source system of FIG. 23 .

FIG. 24B shows two example sequentially filtered EPCS light beams, with the top example formed using four filter wavelengths and the bottom example formed using two filter wavelengths as formed using the remote light source of FIG. 23 .

FIGS. 25A through 25C illustrates example configurations of the EPCS detector system for the enhanced EPCS sub-system.

FIG. 26 is similar to FIG. 4A and illustrates an example of an enhanced LSP sub-system.

FIG. 27A is a schematic diagram of an example power monitoring system that can be implemented in the enhanced LSP sub-system of FIG. 26 .

FIG. 27B is a plot of the (normalized) optical power OP versus polarization angle (degrees) for transmitted and reflected portions of the light beam for the power monitoring system of FIG. 27A.

FIG. 28 is a plot of the central tension CT (MPa) versus the lens position LP (mm) (relative to a reference position) for the axially movable focusing lens, showing a variation in the CT with lens position and thus illustrating the importance of providing a proper focus for the focused LSP light beam.

FIG. 29 is similar to FIG. 11B and shows measurement parameters for the LSP detector system relative to the CS substrate for performing a calibration process.

FIG. 30A is a plot of the intensity I(p) versus pixel location p(x) in the x-direction along an LSP beam for a select depth in the CS substrate and showing an example of a tilted Gaussian fit to the measured intensity to estimate the center of the LSP light beam as indicated by the large black dot at the peak of the fitted curve (solid line).

FIG. 30B is a plot of the pixel locations p(x) and p(y) of the peak centers as determined by the method associated with FIG. 30A.

FIG. 30C is a schematic diagram similar to FIG. 10C that shows an LSP image (intensity contour) with two superimposed fit lines (FL1 and FL2) running through the center of the two line-image sections, with the intersection of the two fitted lines representing the center point of the LSP image.

FIG. 30D is a plot of the LSP image intensity I(p) vs. pixel position p along the fitted line FL2 and in the vicinity of the entrance point EP and illustrates an example edge intensity profile having a maximum intensity I_(MAX), a minimum intensity I_(MIN), and the half-maximum intensity I_(1/2), which resides midway between I_(MAX) and I_(MIN) and defines the position (pixel location) of an entrance point ENP, which in the example plot is about at pixel 32.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute part of this Detailed Description.

Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation.

In some parts of the discussion, the z coordinate is used for the depth direction into the substrate, while in other parts of the discussion a different coordinate is used.

The acronym “IOX” stands for “ion exchange” or “ion exchanged,” depending on the context of the discussion.

The acronym “CS” when used to described a type of substrate (as in “CS substrate”) means “chemically strengthened.” The acronym CS can also mean “compressive stress,” and which meaning is being used for this acronym will be apparent by the context of the discussion.

The term “strengthened” for the CS substrates considered herein means that the original CS substrates have undergone a process to create some stress profiles that could have a variety of shapes, typically intended to make the CS substrates stronger and thus harder to break. Example strengthening processes include ion exchange, tempering, annealing and like thermal processes.

The term “transparent” as used in reference to the CS substrate means a CS substrate that has sufficient optical transmission at the give measurement wavelength (i.e., the EPCS wavelength λ_(A) or the LSP wavelength λ_(B)) to make the satisfactory measurement (i.e., an EPCS measurement or a LSP measurement) of the CS substrate that yields a sufficiently accurate measurement of the stress characteristics associated with the given measurement.

The abbreviation “ms” stands for “millisecond.”

The abbreviation “nm” stands for “nanometer.”

The term “near-surface” such as when referring to a near-surface waveguide or a near-surface spike region of the CS substrate, refers to the portion of the substrate body that resides immediately adjacent a given surface (e.g., the top or measurement surface) of the CS substrate.

In an example, a glass-based substrate is used to form the CS substrate. The term “glass-based substrate” as used herein includes any object made wholly or partly of glass, such as laminates of glass and non-glass materials, laminates of glass and crystalline materials, and glass-ceramics (including an amorphous phase and a crystalline phase). Thus, in an example, the glass-based CS substrate” can consist entirely of a glass material while in another example can consist entirely of a glass-ceramic material.

The terms “image” and “line image” are used herein to describe a distribution of light (i.e., intensity distribution) of a portion of the X-shaped LSP image as formed by scattered light at digital detector (CCD camera or CMOS sensor, etc.) by the LSP sub-system, and an imaging system is not necessary to form the LSP image as considered herein.

In the discussion below, the LSP sub-system is configured to cycle between two or more polarization states (or just “polarizations” for short). In an example, there can be up to eight different polarization states per cycle that combine the linear, elliptical and circular polarizations as is known in the art.

The term “stress” as used herein can generally mean compressive stress or tensile stress. In the plots of FIGS. 15B, 16B, 19B and 19D, the compressive stress is negative while the tensile strength is positive. Whether the stress is compressive or tensile depends on the location or depth region of the CS substrate under consideration. A positive value for the compressive stress is understood to mean the magnitude of the compressive stress. The stress is denoted by S or by σ and is taken to refer to the compressive stress unless otherwise noted or as otherwise understood by the context of the discussion. In some instances, the compressive stress is denoted CS, such as for the knee stress CS_(k). The stress profile is the stress S as a function of depth into the CS substrate, and the depth coordinate can be any local coordinate, and in the discussion below both z and x are used as the local coordinate.

In an example, the “characterizing” of a CS substrate includes determining one or more stress-based properties of the CS substrate, such as a stress profile S(z), a depth of layer DOL, a surface stress S(0), a depth of compression DOC, a central tension CT, and a birefringence profile B(z). In an example, the characterizing utilizes both EPCS and LSP measurements that respectively provide first and second stress characteristics that when combined provide a “full characterization” of the stress characteristics of the CS substrate, wherein the term “full characterization” means a more complete characterization of the stress and stress-related properties than is possible with just the first stress characteristics of the EPCS measurements or just the second stress characteristics of the LSP measurements.

The acronym “OR” stands for “optical retardation” and is measured in radians (“rads”) unless otherwise stated. Plots of the optical retardation versus depth into the CS substrate are referred to below as “OR vs. D” curves or plots, where D is understood as being the depth into the CS substrate body from the top (measurement) surface.

The term “index-matching fluid” means a fluid with a refractive index substantially the same as another material to facilitate optical coupling. In an example, the index-matching fluid comprises an oil or a mixture of oils. The refractive index of the index-matching fluid is denoted by either n_(ƒ) or n_(oil), i.e., these two expressions are used interchangeably below.

The term “plate” and “waveplate” can used interchangeably when referring to types of waveplates (e.g., a half-wave plate is the same as a half-wave waveplate, etc.). To avoid confusion, the term “waveplate” is referred below to below as a “plate” when the proceeding term includes the word “wave.”

The term “sub-system” is used in places where convenient to distinguish from a larger system in which a smaller system resides. Thus, the terms “system” and “sub-system” are used interchangeably for ease of discussion.

The CS Substrate

FIG. 1A is an elevated view of an example type of CS substrate 10 in the form of a planar sheet. The CS substrate 10 has a body 11, a top surface 12, a bottom surface 14 and sides 16. The CS substrate 10 has a thickness TH and a mid-plane MP half way between the top surface 12 and bottom surface 14 and parallel thereto.

In some cases, the thickness TH can be in the range from 0.050 mm TH 2 mm, such as 0.20 mm≤TH≤2 mm, 0.25 mm≤TH≤2 mm, 0.3 mm≤TH≤2 mm, or 0.3 mm≤TH≤1 mm, and any and all sub-ranges formed between these endpoints.

Example types of CS substrates 10 are glass-based and are used as protective covers for displays and/or housings for mobile devices such as smart phones, tablets, laptop computers, GPS devices, etc. Such CS substrates 10 tend to be thin and planar, such as shown in FIG. 1A.

The CS substrate 10 includes a near-surface waveguide (NSWG) 18 that resides in the body 11 proximate the top surface 12. In an example, the NSWG 18 is formed using an IOX process and is defined by at least one IOX region of varying refractive index.

FIG. 1B is a plot of the refractive index n versus the depth z into the CS substrate for an example NSWG 18. The surface refractive index is denoted n_(s) while the bulk refractive index, i.e., the refractive index of the substrate material that has not been affected by the chemical strengthening process is denoted n_(B).

The plot of FIG. 1B shows an example refractive index profile n(z) that defines two (IOX) regions, namely a first near-surface spike region R1 and a second deep region R2. There is also a third region R3 deeper than the second deep region and it is referred to herein as the “bulk” region having the refractive index n_(B). The near-surface spike region R1 has a maximum refractive index n_(s) at the surface and a rapid decrease in the refractive index with depth (z) to a value n_(k) over a relatively shallow depth z=D1 that defines a first “spike” depth of layer DOL_(sp). The deep region R2 has a slower decrease in the refractive index from n_(k) down to a depth D2 that defines a total depth of layer DOL_(T) where the third bulk region R3 starts. The first and second regions R1 and R2 meet at (and thus define) a knee KN at z=z_(k) where as noted above the refractive index n=n_(k) and which is associated with a knee (compressive) stress CS_(k).

Because of the two distinct refractive-index regions R1 and R2 in the NSWG 18, certain guided modes propagate only in the uppermost spike region R1 while other guided modes travel in both regions R1 and R2, while still other guided modes travel only in the deep region R2. Other refractive index profiles n(z) include more uniform changes in the refractive index. Some of the deep guided modes can extend into the bulk region R3.

The refractive index profile n(z) of FIG. 1B can be formed by a dual IOX (DIOX) process where one IOX process forms the deep region R2 and another IOX process different from the first IOX process forms the spike region R1. The plot of FIG. 1B is representative of a DIOX process carried out in Li-containing glass CS substrate 10 wherein Li ions are exchanged with potassium and sodium ions in two distinct IOX processes, with the potassium IOX process generating the spike region R1.

The Hybrid EPCS-LSP System

FIG. 2A is a schematic diagram of the hybrid EPCS-LSP measurement system (“hybrid system”) 20 as disclosed herein shown along with an example CS substrate 10. The hybrid system 20 includes a coupling prism assembly 40, an EPCS measurement sub-system (“EPCS sub-system”) 100, a LSP measurement sub-system (“LSP sub-system”) 200, and a system controller 400. The coupling prism assembly 40 defines a measurement location ML on the CS substrate 10.

The EPCS sub-system 100 generates a EPCS measurement signal SA representative of first stress characteristics of the CS substrate at the measurement location ML as embodied in a mode spectrum of the guided modes of the NSWG 18. The first stress characteristics can include one or more of the following: a surface compressive stress S(0), a total depth of layer DOL_(T), a spike depth of layer DOL_(sp), a knee stress CS_(k) and a birefringence B.

The LSP sub-system 200 generates a LSP measurement signal SB representative of second stress characteristics of the CS substrate at the measurement location ML as embodied in optical retardation (OR) information as a function of depth into the CS substrate, including the deep region R2. The second stress characteristics can include one or more of the following: a stress profile, a depth of compression DOC and a central tension CT.

In an example, EPCS and LSP measurements of the first and second stress characteristics are made without moving the measurement location ML. In another example, the EPCS and LSP measurements of the first and second stress characteristics are made by translating the coupling prism assembly 40 such that the EPCS and LSP measurements are made at the same position on the substrate rather than at slightly spaced part positions at the measurement location as defined by the configuration of the coupling prism assembly.

In an example, the EPCS and LSP measurements of the first and second stress characteristics are made without removing either the coupling prism assembly 40 or the CS substrate 10 from the measurement location ML. This represents an advantage over the prior art in that both EPCS and LSP measurements can be made in a single system without having to remove or otherwise handle the CS substrate to bring it to a different measurement system.

The EPCS and LSP measurement signals SA and SB are sent to the system controller 400 for processing. The system controller 400 can comprise for example a micro-controller, computer, programmable logic controller (PLC), etc. In an example, the system controller 400 is configured with instructions embodied in a non-transitory computer-readable medium (e.g., software) to control the operation of the hybrid system 20 and perform the calculations for determining the first and second stress characteristic of the CS substrate 10 based on the EPCS and LSP measurement signals SA and SB.

In an example, the system controller 400 processes the EPCS and LSP measurement signals SA and SB to define a stress profile and related stress characteristics from the top surface 12 of the CS substrate 10 down to at least the bottom of the deep region R2. In other words, the system controller combines the first and second stress characteristics obtained from EPCS sub-system 100 and the LSP sub-system 200 to generate a more complete or “full” stress profile of the CS substrate than is possible with just one of the measurement sub-systems.

The coupling prism assembly 40 includes a EPCS coupling prism 42A and a LSP coupling prism 42B operably supported by a prism support structure 46. The coupling prism assembly 40 is operably disposed on or proximate the top surface 12 of the CS substrate 10. In examples discussed below, the EPCS coupling prism 42A and the LSP coupling prism 42B can be separate coupling prisms or different sections of a single (common) coupling prism.

With continuing reference to FIG. 2A, the hybrid system 20 includes an example housing 21 having dimensions L1 and L2. Example dimensions for L1 and L2 are in the range from 8 inches to 12 inches for a relatively compact embodiment of the hybrid system 20.

The EPCS sub-system 100 includes a EPCS light source system 110 and a EPCS detector system 140 optically coupled via the EPCS coupling prism 42A. The LSP sub-system 200 includes a LSP light source system 210, an optical compensator 230 and a LSP detector system 240 optically coupled to the optical compensator via the LSP coupling prism 42B. The EPCS and LSP detector systems 140 and 240 are operably connected to the system controller 400. Examples of the EPCS sub-system 100 is described in U.S. Pat. No. 9,534,981 and in U.S. Pat. No. 9,696,207, which are incorporated by reference herein. Examples of the LSP sub-system 200 are described in U.S. Pat. No. 4,655,589, and in U.S. Provisional Patent Application Ser. No. 62/753,388, which are incorporated by reference herein.

FIG. 2B is a more detailed schematic diagram of the hybrid EPCS-LSP system of FIG. 2A, showing example configurations for the EPCS sub-system 100 and the LSP sub-system 200. FIG. 3A is a schematic diagram of example EPCS sub-system 100. FIGS. 4A through 4C are schematic diagrams of example LSP sub-systems 200.

The EPCS Sub-System

With reference to FIG. 2B and FIG. 3A, the EPCS light source system 110 of the EPCS sub-system 100 includes a EPCS light source 112 that generates a EPCS light beam 116 at a first wavelength λ_(A) along a first axis A1. The first wavelength λ_(A) can also be referred to as the EPCS wavelength.

The EPCS light source system 110 also includes along the first optical axis A1: an optional polarizer 118, a light diffuser 122 that resides downstream of the EPCS light source 112, and a focusing lens 120 that resides downstream of the light diffuser. In an example, the light source comprises a light-emitting diode (LED), and further in an example the LED operates at an EPCS measurement wavelength λ_(A) of 365 nm. The EPCS detector system 140 resides along a second axis A2 and includes in order along the second axis: a focusing lens 142, a band-pass filter 144 centered on the wavelength λ_(A), an attenuator 146, a TM-TE polarizer 148 (which has TM and TE sections, not shown) and a digital detector (e.g., a digital camera, image sensor, CCD array, etc.) 150 that has TM and TE sections (not shown) as defined by the TM-TE polarizer 148.

The EPCS light beam 116 from the EPCS light source 112 is diffused by the light diffuser 122 and is focused by the focusing lens 120 to form a focused EPCS light beam 116F. The focused EPCS light beam 116F is incident upon the EPCS coupling prism 42A at an input surface 43A. This couples the EPCS focused light beam into the NSWG 18 at a first (EPCS) coupling interface INT1 defined by the top surface 12 of the CS substrate and a bottom or “coupling” surface 45A of the EPCS coupling prism 42A. The first coupling interface INT1 can include an index-matching fluid 5A, as discussed in greater detail below.

A reflected EPCS light beam 116R is formed from the focused EPCS light beam 116F at the first EPCS coupling interface INT1 and exits the output surface 44A of the EPCS coupling prism 42A to travel along a second axis A2. The first and second axes A1 and A2 residing in a common plane (e.g., x-z plane of FIG. 3A). The reflected EPCS light beam 116R includes information about the mode spectrum of the guided modes of the NSWG 18. The reflected EPCS light beam 116R is focused by the focusing lens 142 in the EPCS detector system 140 to form an image of the mode spectrum of the guided light at the EPCS digital detector 150.

The band-pass filter 144 assures that only the reflected EPCS light beam 116R makes it through to the EPCS digital detector 150. The attenuator 146 assures that the detected reflected EPCS light beam 116R has the appropriate intensity distribution for efficient digital detection. The TM-TE polarizer 148 defines TM and TE sections for the digital detector so that TM and TE mode spectra can be captured by the EPCS digital detector 150. The TM and TE mode spectra are embodied in the first detector signal SA sent to the system controller 400 for processing. It is noted that the order of the band-pass filter 144, the attenuator 146 and the focusing lens 142 is not critical and is intentionally shown as being different between FIGS. 2B and 3A to illustrate this point.

FIG. 3B is a schematic representation of an idealized mode spectrum 160 as captured by the EPCS digital detector 150. Local (x,y) Cartesian coordinates are shown for reference. The mode spectrum 160 has TM and TE total-internal-reflection (TIR) sections 161TM and 161TE respectively associated with TM and TE guided modes, and non-TIR sections 162TM and TE respectively associated with TM and TE radiation modes and leaky modes. The TIR section 161TM includes one or more TM “fringe” or TM mode lines 163TM while the TIR section 161TE includes one or more TE “fringes” or TE mode lines 163TE. The TM and TE mode lines 163TM and 163TE are generally aligned in the x direction and are spaced apart in the y direction.

Transition regions (“transitions”) 166TM and 166TE between the TIR section 161TM, 161TE and the non-TIR sections 162TM, 162TE define a critical angle for the optical coupling into and out of the NSWG 18 of the CS substrate 10 for TM and TE polarized light, and are referred to as the critical angle transitions. The difference in locations of the start of the critical angle transitions 166TM and 166TE is proportional to the knee stress CS_(k) and this is proportionality is indicated by “˜CS_(k)” in FIG. 3B.

The TM and TE mode lines 163TM and 163TE can either be bright lines or dark lines, depending on the configuration of the EPCS sub-system 100. In FIG. 3B, the TM and TE mode lines 163TM and 163TE are shown as dark lines for ease of illustration.

The stress characteristics for the EPCS measurement are calculated based on the difference in the y positions of the TM and TE mode lines 163TM and 163TE in the mode spectrum 160. The birefringence B is the difference between the effective indices of the TM and TE polarizations, wherein the effective indices are represented by the y positions of the mode lines. The surface compressive stress S(0)=CS is computed by the y distances between the mode lines (effective indices) and the ratio B/SOC, where SOC is the stress optic coefficient. At least two TM and TE mode lines 163TM and 163TE are needed to calculate the surface stress S(0). Additional mode lines are needed to calculate the compressive stress profile S(z). The depth of layer DOL_(T) is a measure of stress penetration or ion penetration length into the body 11 of the CS substrate 10, and in the case of an IOX process, can also be calculated by the y-locations and number of mode lines 163TM and 163TE. The TM and TE mode line locations along the y axis are thus the most basic measurement for inferring stress-related characteristics of the CS substrate 10. The calculations for determining the stress characteristics of the CS substrate 10 based on the EPCS measurements using the EPCS sub-system 100 are carried out in the system controller 400.

The LSP Sub-System

With reference now to FIG. 2B and FIGS. 4A through 4C, the LSP light source system 210 of the LSP sub-system 200 includes a LSP light source 212 that generates a LSP light beam 216 of wavelength λ_(B) along a third axis A3. In an example, the LSP light source 212 is configured as a laser diode that operates at a second wavelength λ_(B)=415 nm. The second wavelength λ_(B) can also be referred to as the LSP wavelength.

The LSP light source system 210 includes in order along the third axis A3: an optional neutral density filter 218 (shown in FIGS. 2B and 4A), a first focusing lens 220, a movable light diffuser 222, and a second focusing lens 224. The movable light diffuser 222 can comprise a holographic element configured to perform light diffusion at the wavelength λ_(B). In an example, the movable light diffuser can comprise rotating light diffuser or oscillating light diffuser. One or more fold mirrors FM can be used to fold the LSP sub-system 200 to make it more compact.

The optical compensator 230 resides along the (folded) third axis A3 and includes a polarizer, which can be in the form of a polarizing beam splitter PBS 232. The optical compensator 230 also includes a half-wave plate 234H and a quarter-wave plate 234Q with one of the wave plates being rotatable relative to the other to change the state of polarization of the LSP light beam 216. In an example, the optical compensator 230 can comprise an electronically controlled polarization modulator, such as a liquid-crystal-based modulator or a ferroelectric liquid-crystal-based modulator or like modulator.

In an example, the optical compensator 230 is operably connected to or otherwise includes a controller (not shown) that controls the polarization switching operation performed by the optical compensator. In an example, the optical compensator 230 can comprise a single liquid crystal device. In another example, the optical compensator 230 can comprise multiple elements such as polarizers, wave plates, filters, prisms (e.g., wedge prisms), etc. In an example, the optical compensator 230 causes the LSP light beam 216 to go through a full polarization cycle (i.e., change between two or more select polarizations) in anywhere from less than 1 second to 10 seconds. In an example, the optical compensator 230 can be operably connected to and controlled by the system controller 400.

A third focusing lens 236 resides downstream of the optical compensator 230 and is used to form a focused LSP light beam 216F, which is directed to the LSP coupling prism 42B. The LSP coupling prism has respective input and output surfaces 43B and 44B and a bottom or “coupling” surface 45B. The coupling surface 45B and the top surface 12 of the CS substrate 10 defines a second (LSP) coupling interface IF2. In an example, the second coupling interface INT2 includes an index matching fluid 5B, as discussed below.

The LSP detector system 240 resides along a fourth axis A4 that is orthogonal to the third axis A3, i.e., the fourth axis A4 resides in the Y-Z plane.

In an example, the LSP detector system 240 includes a collection optical system 243 and a digital detector (e.g., a CCD camera) 246. In an example, the collection optical system 243 is telecentric and has unit magnification. The LSP detector system 240 can also include a bandpass filter 244 centered on the second wavelength λ_(B). In the example shown in FIG. 4C, the digital detector 246 comprises array of imaging pixels 247, which in an example can have a dimension of between 1.8 microns and 10 microns.

In the operation of the LSP sub-system 200, the focused LSP light beam 216F is incident upon the input surface 43B of the LSP coupling prism 42B and travels to the coupling surface 45B and then through the index-matching fluid 5B and to the top surface 12 of the CS substrate 10 to enter the body 11 of the CS substrate. The focused LSP light beam 216F has a select polarization at any given time as defined by the optical compensator 230. The (polarized) input focused LSP light beam 216F is scattered by stress-induced features in the body 11 of the CS substrate 10 to form a scattered LSP light beam 216S. The scattered LSP light beam 216S exits the CS substrate 10 at the top surface 12, passes back through the second coupling interface INT2 and then exits the LSP coupling prism 42B at the output surface 44B. The scattered LSP light beam 216S travels to the LSP detector system 240 and is directed to the digital detector 246 by the collection optical system 243. The scattered LSP light beam 216S forms a LSP image 248 on the digital detector 246, as shown in the close-up view of FIG. 4D. This defines a digital LSP image. The LSP image 248 as discussed below is taken to be the digital LSP image unless otherwise noted. The characteristic “X” shape of the LSP image 248 is known in the art of LSP and is due to reflections of the scattered LSP light beam 216S from the different interfaces associated with the LSP interface INT2 as defined by CS substrate 10, LSP coupling prism 42B and the index-matching fluid 5B.

As shown in FIG. 4D, the X shape of the LSP image 248 is defined by two crossed line images LI each having a local length coordinate x_(L) along its length. Each line image LI has an intensity distribution I(x_(L)) that is measured by the pixels 247 that coincide with the line image. The digital detector converts the intensity distributions I(x_(L)) to a second detector signal SB, which is sent to the system controller 400. Only one of the line images LI is needed for performing a measurement. In an example, image processing is used to identify a portion of the LSP image 248 for use subsequent processing to extract the optical retardation information, as explained below.

In an example, a given measurement of the CS substrate 10 using the LSP sub-system 200 includes making measurements for a measurement time t_(M) of between 1 second and 10 seconds. During the measurement time t_(M), the polarization state of the LSP light beam 216 varies between the different polarization states, preferably making one or more cycles through the polarization states. Meantime, for each polarization state, the digital detector 246 captures the LSP image 248 during exposure times t_(E). In an example, the exposure times t_(E) are about the same as the frame rate FR of the digital detector 246. An example exposure time t_(E)=50 ms, which corresponds to a frame rate FR=20 frames per second. The exposure time t_(E) can also be less than the frame rate.

The electronically captured LSP images 248 differ in their intensity distributions I(x_(L)) depending on the polarization state of the input focused LSP light beam 216F and the optical retardation incurred along the beam path. The difference is due to the difference in the destructive and constructive interference along the length of the scattered LSP light beam 216S as a function of depth D into the CS substrate 10 between the different polarization states. The differences between the multiple intensity distributions I(x_(L)) for the different polarization states is used by the system controller 400 to calculate an optical retardance OR as a function of depth D into the body 11 of the CS substrate 10 using relationships well known in the art. Likewise, multiple optical retardance curves OR vs the depth D (“OR vs. D plots”) are calculated using the differences in the intensity distributions I(x_(L)). For example, for a 3 second measurement time t_(M) with an image sensor frame rate FR of 20 frames/second, a total of 60 plots of I(x_(L)) vs. D can be generated to compute OR vs. D and used to calculate one or more stress-related characteristics of the CS substrate 10.

While the intensity distributions I(x_(L)) for the LSP image 248 necessarily differ between polarization states of the input light beam 112, when there is stress present in the CS substrate 10, the different OR vs. D curves (plots) as calculated from the measured intensity distributions should ideally be the same for a given CS substrate at the given measurement location for CS substrates where the stress profile is (ideally) constant.

While LSP measurement technique can generate a stress profile S(z), it does not generally produce an accurate representation of the stress profile in the near-surface region of the CS substrate 10. There are least two problematic effects that present challenges for extracting an accurate characterization of the near-surface stress profile for a CS substrate 10 using an LSP measurement from the LSP sub-system 200. One problematic effect is referred to as a “fireball” effect, which is caused by excessive light scattering at the LSP interface INT2. The excessive light scattering generates noise, which corrupts the LSP measurement data for near-surface region, which in an example is the first 60 microns to 100 micrometers below the top surface 12 of the CS substrate 10.

The other problematic effect is caused by the convolution of photons scattered from different depths into the signal corresponding to a particular depth. This convolution significantly changes the signal in the region where stress changes fast, which is usually in the near-surface compression region, most often in the first 80 microns, 100 microns, or 150 microns, but sometimes as high as 200 microns. The region of fast change is thicker for larger thicknesses of Li-based glass)

Some prior-art LSP systems attempt to decrease these convolution effects by using a very focused beam near the CS substrate surface, with beam diameter as small as 10 microns. Unfortunately, this leads to other problems—such as increased laser noise (e.g., speckle) in the same depth region of interest, making the extracted stress profiles in the near-surface region even less reliable.

Coupling Prism Assembly

The hybrid system 20 utilizes the aforementioned coupling prism assembly 40, which operably supports the EPCS coupling prism 42A and the LSP coupling prism 42B to provide the prism coupling for the EPCS sub-system 100 and the LSP sub-system 200 when making the EPCS measurement and the LSP measurement of the CS substrate 10.

FIG. 5A is an elevated view of a top portion of an example coupling prism assembly 40 showing an example support frame 48. FIG. 5B is an elevated view similar to FIG. 5A and that additionally includes a cover plate 60. The example support frame 48 includes a EPCS frame section 48A that supports the EPCS coupling prism 42A and a LSP frame section 48B that supports the LSP coupling prism 42B. The support frame 48 also includes an isolation member 50 disposed between the EPCS frame section 48A and the LSP frame section 48B that is configured to optically isolate the EPCS and LSP coupling prisms 42A and 42B. In an example, the isolation member 50 also prevents the mixing of index-matching fluids 5A and 5B respectively used with the EPCS and LSP coupling prisms 42A and 42B. In another example, the isolation member 50 allows for a single index-matching fluid to be used with both the EPCS and LSP coupling prisms 42A and 42B, i.e., the single index-matching fluid can flow between the first and second interfaces INT1 and INT2 defined by the two different prisms. In one example, the isolation member 50 is a separate part from the support frame 48 and is attached thereto. In another example, the isolation member 50 is part of the support frame 48, i.e., is formed integral therewith during the formation of the support frame.

In an example, the EPCS and LSP frame sections 48B and the isolation member 50 including securing tabs 52 that includes mounting holes 53 that allow for securing the cover plate 60 to the frame sections using securing members (not shown). The cover plate 60 includes a first aperture 62A sized to accommodate the coupling surface 45A of the EPCS coupling prism 42A and a second aperture 62B sized to accommodate the coupling surface 45B of the LSP coupling prism 42B.

FIGS. 6A and 6B illustrate an example method wherein the EPCS and LSP frame sections 48A and 48B are formed using a resin mold process. The process provides for precision alignment of the EPCS and LSP coupling prisms 42A and 42B. In an example, the molding process is carried out with example EPCS and LSP coupling prisms 42A and 42B in place on a stable platform 75. This process is discussed in greater detail below.

FIG. 7 is an x-z cross-sectional view of an example prism support structure 46 attached to an example support plenum 70 of the hybrid system 20 using the securing tabs 52 and securing members 54, such as screws, that pass through the mounting holes 53. The support plenum 70 has a top surface 71 and measurement aperture 72. The top surface 71 defines an example measurement plane MP at the measurement aperture 72. The prism support structure 46 is supported by the support plenum 70 such that the EPCS and LSP coupling prisms 42A and 42B reside at the measurement aperture 72. In an example, the EPCS and LSP coupling surfaces 45A and 45B of the EPCS and LSP coupling prisms 42A and 42B reside at or substantially at the measurement plane MP.

In an example, the CS substrate 10 is operably supported by a movable substrate holder 80 that holds the CS substrate over the measurement aperture 72 so that the EPCS and LSP coupling prisms 42A and 42B can be interfaced with the top surface 12 of the CS substrate 10 to establish the first and second coupling interfaces INT1 and INT2 at or substantially at the measurement plane MP. In an example, the movable substrate holder 80 is conveyed over the top surface 71 of the support plenum 70 using conveying elements 73 such as rollers, wheels, sliders, bearings, etc. In an example, the CS substrate 10 is supported by the movable substrate holder 80 at an interior lip 82 that supports an outer (perimeter) portion of the top surface 12 of the CS substrate. In an example, the plane of the interior lip 82 defines an example measurement plane MP. Thus, FIG. 7 shows two different example locations of the measurement plane MP.

FIG. 8A is an elevated view that illustrates an example wherein the support plenum 70 is in the form of a plate that includes pressure-vacuum (PV) elements or PV conduits 90 (e.g., PV bars) used to pneumatically engage the CS substrate 10 to pull the CS substrate onto the coupling surfaces 45A and 45B of the EPCS and LSP coupling prisms 42A and 42B via vacuum (negative pressure) and then release the CS substrate from the prisms via pressure (positive pressure). FIG. 8B is a cross-sectional view of the support plenum and measurement aperture of the configuration of FIG. 8A showing an example vacuum system 91 that include the PV elements (PV bars) 90 and a vacuum source 92.

Note that the interior lip 82 of the movable substrate holder 80 defines a stop member for limiting the vertical movement of the CS substrate 10 when a vacuum is applied to the CS substrate via the vacuum system 91.

Hybrid System Employing a Single Index-Matching Fluid

An example embodiment of the hybrid system 20 such as shown in FIG. 6D employs a single index-matching fluid 5 of refractive index n_(ƒ) for both EPCS and LSP sub-systems 100 and 200. This is a counter-intuitive approach since a single index-matching fluid 5 would typically be considered as not being able to produce good measurement results from both sub-systems at the same time for at least the following reasons.

If the index-matching fluid is chosen based on EPCS measurement considerations, the index matching fluid has a refractive index n_(ƒ) that is substantially higher (e.g., by 0.1 or more) than the surface refractive index n_(s) of CS substrate to facilitate coupling of light into the guided modes and to obtain good fringe contrast in the captured TM and TE mode spectra.

On the other hand, this level of refractive index contrast (difference) Δn between refractive indices of the index-matching fluid 5 and the top surface 12 of the CS substrate 10 causes significant surface scattering at the coupling interface INT2 from beam deflection at the index mismatch associated with the micro-roughness of the surfaces. This is problematic for the accurate extraction of retardation and stress measurements at moderate depths based on receiving and processing scattered light from the CS substrate. The high degree surface scattering produces a “fireball”, e.g., a large bright spot on the image of the scattered light beam where the pixels 247 of the digital detector (CCD camera) 246 are saturated with photons. This results in the loss of a substantial amount of stress-related information. Well-polished surfaces or pristine surfaces (such as formed by fusion drawing) tend to have less scattering.

If the index-matching fluid n_(ƒ) is approximately matched to (e.g., being similar, slightly higher, or slightly lower than) the surface refractive index n_(s) of the CS substrate 10 to ensure low surface scattering, the fringe contrast in the mode spectrum 160 is usually poor when there is a steep change in the refractive index near the surface, e.g., such as the spike region R1 (see FIG. 1B) as caused by a shallow concentrated spike of K₂O concentration as produced by an IOX process. Furthermore, the position and contrast of the TM and TE fringes 163TM and 163TE become dependent on the thickness of the index-matching fluid. These two effects make it very difficult to measure the surface CS and spike DOL accurately using the EPCS sub-system 100.

If the index-matching fluid 5 is chosen to have a refractive index n_(ƒ) lower than the substrate (bulk) refractive index n_(B) of the CS substrate 10 (which also usually means lower than the surface refractive index), then the thickness of the index-matching fluid must be very small (e.g., less than 0.4 micron) to enable light coupling into the waveguide modes of the near-surface portion (spike region R1) of the NSWG 18 for the surface CS measurement. The small thickness is also required for measuring the critical angle for coupling light that travels in the deep region R2 between the surface spike region R1 and the bulk region R3. This is difficult to achieve consistently in a production environment due to issues with small particle contamination. These issues cause problems for accurately measuring the surface (compressive) stress S(0) and the “knee stress” S_(k) at the bottom of the surface refractive-index spike region R1 for dual IOX Li-containing glasses and glass ceramics.

It turns out a single index-matching fluid 5 for both EPCS and LSP measurements can be used under select conditions wherein the spike region R1 of the CS substrate 10 has a normalized slope S_(n)=|(λ/n)dn(z)/dz|<0.0005, or more preferably S_(n)<0.0004, where λ is the measurement wavelength and n(z) is the index of refraction of the CS substrate 10 at the measurement wavelength.

In one embodiment, an index-matching fluid 5 having a refractive index n_(ƒ) that is higher (greater) than the surface refractive index n_(s) of the CS substrate 10 glass by amounts Δn=n_(ƒ)−n_(s) in the range from 0.02 to 0.06 is found to produce adequate measurement results for both the EPCS and LSP measurements. When S_(n)<0.0004, it is preferred that Δn be in the high end of the above-stated range, e.g. from 0.05 to 0.06.

In one aspect of the invention, the measurement wavelength λ_(A) for the EPCS measurement is reduced to reduce the normalized slope S_(n) to more readily satisfy the above-described conditions. In one example, the measurement wavelength λ_(A) of the EPCS measurement is shorter than the measurement wavelength λ_(B) Of the LSP measurement by 5% or more, to help achieve a smaller normalized slope Sn. In an example, one or more light blocks (not shown) can be selectively positioned on the beam path of the EPCS sub-system 100 to preferentially block light rays propagating at larger incidence angle corresponding to higher effective indices. This enhances the contrast of the captured TM and TE fringes of the guided modes for the near-surface spike region R1 of the NSWG 18.

In another embodiment, the surface spike region R1 may have normalized slope S_(n)>0.0005. In an example, the index-matching fluid may be selected to have at the EPCS measurement wavelength λ_(B) a refractive index n_(ƒ) very close to the effective refractive index at the z location z_(k) at the knee KN, i.e., at the bottom of the spike region R1. In this case, n_(ƒ)=n_(crit), where n_(crit) is the refractive index associated with the critical angle of the spike region, i.e., the angle below which light does not travel as a guided wave within the spike region R1

In many cases of practical interest, the difference in effective index between the TM and the TE guided wave at the location corresponding to the bottom of the surface spike region R1 is relatively small. For example, in most cases of practical interest the difference is less than 0.0006 refractive-index units (RIU), and most often it is between 0.00015 and 0.0005 RIU. In an example, 0.0001≤|n _(crit) ^(TM) −n _(crit) ^(TE)|≤0.0006.

In some examples, it is adequate to specify that n_(ƒ)=n_(crit), meaning that n_(oil)≈n_(crit) ^(TM) and/or n_(oil)≈n_(crit) ^(TE). To be more specific, n_(ƒ) is not substantially smaller than the smaller of the TM and TE critical indices, and would also not be significantly greater than the largest of the TM and TE critical index. Thus, in an example (and to express the above mathematically): min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.001≤n _(oil)≤max(n _(crit) ^(TM) ,n _(crit) ^(TE))+0.001

The upper limit in the equation immediately above is defined to reduce the chances of missing a fringe associated with the spike region R1 by making n_(ƒ) greater than the effective index of that fringe when the index-matching fluid is absent. Thus, to enable proper accounting for all modes for the purposes of accurately calculating of the depth of the surface spike region R1 (which in an example is defined by a potassium IOX process), it is preferred that n_(ƒ) be not significantly greater than the larger of the TM and TE critical indices n_(knee) ^(TM), n_(knee) ^(TE), but also ideally not significantly greater than the smaller of the two critical indices.

In one embodiment, the mode fringes in the TM and TE mode spectra associated with the spike region R1 are spaced far apart in effective index space, e.g., by more than 0.0015 RIU or preferably by more than 0.002 RIU or even more preferably by than 0.0025 RIU when there is significant effective-index difference between the effective index of the last fringe in a specific polarization state (TM or TE), and the corresponding critical index (n_(crit) ^(TM) or n_(crit) ^(TE)). In this embodiment, the index-matching fluid refractive index n_(oil) may be chosen closer to the higher of the two critical indices, and possibly higher than the greater one of them: max(n _(knee) ^(TM) ,n _(knee) ^(TE))−0.0005≤n _(oil)≤max(n _(knee) ^(TM) ,n _(knee) ^(TE))+0.001 or max(n _(knee) ^(TM) ,n _(knee) ^(TE))−0.0005≤n _(oil)≤max(n _(knee) ^(TM) ,n _(knee) ^(TE))+0.0005

These differences in effective index are easy to establish using the EPCS sub-system 100 by measuring the difference in locations of the critical angles, which correspond to the intensity transitions 166TM and 166TE from bright total-internal reflection to dark (partial reflection) on the sensor, and/or fringe positions, and taking into account the calibration of the instrument (angle per RIU, or pixels per RIU, or spacing of points on the sensor plane per RIU).

In another embodiment having a more general application, the refractive index n_(oil) of the index-matching fluid is chosen closer to the lower of the TM and TE effective indices. This enables the capture of TM and TE fringes that may be close in effective index to the critical index, but may require relatively close proximity between the coupling surface 45A of the EPCS coupling prism 42A and the top surface 12 of the CS substrate 10 (e.g., a few wavelengths).

More specifically, in this embodiment it is preferred that min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.001≤n _(oil)≤max(n _(crit) ^(TM) ,n _(crit) ^(TE)) or min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.001≤n _(oil)≤min(n _(crit) ^(TM) ,n _(crit) ^(TE))+0.0005.

Furthermore, to reduce significant change in the shape of the critical-angle transition, it may be preferred that min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.0005≤n _(oil)≤min(n _(crit) ^(TM) ,n _(crit) ^(TE))+0.0005 or even that min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.0002≤n _(oil)

In some cases of practical interest, the guided mode having the lowest effective index has its effective index very close to that of the critical index, within about 0.0002 RIU. In this case, it may be ideal to also impose a stricter requirement for the index-matching fluid refractive index to be limited from above: n _(oil)≤min(n _(crit) ^(TM) ,n _(crit) ^(TE))+0.0002

In cases where n_(oil) is smaller than at least one of the two critical indices, obtaining a high-contrast transition for proper measurement of the critical index n_(crit) may require the aforementioned close proximity between the coupling surface 45A of the EPCS coupling prism 42A and the top surface 12 of the CS substrate 10 (e.g., a few wavelengths). In an example, this close proximity is enabled by use of a vacuum system attracting the specimen toward the prism via the PV conduits 90 pneumatically connected to the vacuum source 92.

In another embodiment, a correction is made for a systematic error in the calculation of surface compressive stress S(0)=CS when the index-matching fluid refractive index n_(oil) is not significantly different from the effective indices of the guided optical modes that are used to calculate the surface compressive stress CS. In particular, such correction may be preferable to utilize when: min(n _(crit) ^(TM) ,n _(crit) ^(TE))−0.01≤n _(oil)≤max(n _(crit) ^(TM) ,n _(crit) ^(TE))+0.01

In one example embodiment, the correction is prescribed by calibrating the systematic error, e.g., by comparison of the surface compressive stress CS measured using the preferred inventive dual-use index-matching fluid with the CS measured by using a more convention index matching fluid having a relatively large refractive index n_(oil), such as an oil with n_(oil)=1.72 at λ_(A)=590 nm used for measuring a CS substrate 10 having a bulk refractive index n_(B) in the range 1.45 to 1.55.

In a related embodiment, the systematic error may also be calibrated against the breadth of the TM and TE fringes 163TM and 163TE, as the breadth may be associated with the thickness of the index-matching fluid, and at the same time associated with the amount of systematic error in the measurement of the surface compressive stress CS. It should be noted that the systematic error would also depend on the index slope S_(n) of the refractive index profile of the surface spike region R1 of the CS substrate 10. This means that the systematic error can be defined for a particular type of CS substrate having a surface index slope S_(n) that falls within a relatively narrow range. Such a narrow range is typical for CS substrates that employ Li-based glass that has been strengthened using an IOX process.

Hybrid System Employing Two Different Index-Matching Fluids

An example embodiment of the hybrid system 20 employs two different index-matching fluids 5 (denoted 5A and 5B) for the EPCS and LSP sub-systems 100 and 200 respectively, with the two different index-matching fluids 5A and 5B having respective refractive indices n_(fA) and n_(fB) (or n_(oil-A) and n_(oil-B)).

Employing two different index-matching fluids 5A and 5B calls for keeping the two index-matching fluids separated so that they do not mix with each other. In one example discussed above in connection with FIGS. 5A and 5B, the prism support structure 46 includes an isolation member 50 disposed between the EPCS coupling prism 42A and the LSP coupling prism 42B to keep the two index-matching fluids 5A and 5B fluidly separated, i.e., in fluid isolation from one another.

In another embodiment, a pressurized gas (e.g., air) is introduced into a small gap between the EPCS coupling prism 42A and the LSP coupling prism 42B to define an “air curtain” 30 (see FIG. 2B) that ensures that the index-matching fluids 5A and 5B do not interact with other while the CS substrate 10 is being measured in the hybrid system 20. This separation will then enable automatic dripping of the respective index-matching fluids 5A and 5B onto their respective EPCS and LSP coupling prisms 42A and 42B at the same time, thus allowing simultaneous measurements. In an example, the air curtain 30 can be formed using the vacuum system 91 (see e.g., FIG. 8B).

Hybrid System with Reduced Cross-Talk

Given the proximity of the EPCS coupling prism 42A and the LSP coupling prism 42B, cross-talk between the EPCS sub-system 100 and the LSP sub-system 200 can occur. Such cross-talk can reduce the accuracy of the stress measurement of each sub-system. The various embodiments for reducing (including eliminating) cross-talk described below can be used separately or in combination.

In one example, the EPCS detector system 140 for the EPCS sub-system 100 includes the aforementioned band-pass filter 144 centered on the EPCS measurement wavelength λ_(A). Meanwhile, the LSP detector system 240 for the LSP sub-system 200 includes a band-pass filter 244 centered on the LSP measurement wavelength λ_(B). In an example, the respective bandwidths of the band-pass filters 144 and 244 are sufficiently narrow to substantially filter out the other sub-systems measurement wavelength. Since band pass filters can be made very narrow, (e.g., a few nanometers), just a small difference in the measurement wavelengths (e.g., 10 nm) would be more than sufficient to reduce or eliminate cross-talk using the band pass filters. In an example, the given band pass filter can be inserted anywhere between the corresponding coupling prism and detector system.

In another embodiment, a barrier that is optically opaque to the measurement EPCS and LSP wavelengths λ_(A) and λ_(B) is disposed between the EPCS coupling prism 42A and the LSP coupling prism 42B. In an example, the barrier takes the form of the isolation member 50 as discussed above in connection with FIG. 5A. The isolation member 50 can be formed of a rigid material such as aluminum, or a non-rigid material such as rubber, as long as it is capable of stopping EPCS and LSP measurement light from communicating between the EPCS and LSP coupling prisms. As noted above, the isolation member 50 can also be configured to serve the dual purpose of optical isolation and fluid isolation.

Coupling Prism Alignment

The hybrid system 20 provides the most accurate measurements when the EPCS and LSP coupling prisms 42A and 42B are aligned with respect to each other and with their coupling surfaces 45A and 45B residing in a common plane.

To achieve such alignment, the coupling prism assembly 40 employs the aforementioned prism support structure 46 is used. In an example of forming the prism support structure 46, the coupling surfaces 45A and 45B of the EPCS and LSP coupling prisms 42A and 42B are first ground and polished to a high degree of flatness and perpendicularity. With reference again to FIG. 6A, the EPCS and LSP coupling prisms 42A and 42B are then placed on a stable platform 75, such as a precision flat granite bar, with the coupling surfaces 45A and 45B resting upon a surface 76 of the stable platform.

With reference now to FIG. 6B, a mold 49 is installed on the stable platform 75 at the surface 76 and a resin 49R is then poured into the mold. Upon hardening of the resin, the walls of the mold 49 are removed to define the prism support structure 46 of the coupling prism assembly 40, such as shown in FIG. 5B. In an example, the molded prism support structure 46 includes the isolation member 50 in the form of a thin wall 47 between the EPCS and LSP coupling prisms 42A and 42B, as shown in FIG. 6C. In an example, the molded prism support structure 46 is formed such that at least one of the prisms is partially encased to avoid cross-talk. In an example, the molded prism support structure 46 comprises or consists of a unitary molded structure, i.e., is a single piece made of a single material (i.e., the piece monolithic) and so is not formed by joining two or more components.

In an example, the molded prism support structure 46 includes securing tabs 52 that include mounting holes 53 for securing the prism support structure 46 to the support plenum 70 (see also FIG. 5A). The use of the movable substrate holder 80 as shown in FIG. 7 and described above enables EPCS and LSP measurements to be made at the same location on the CS substrate 10. The movable substrate holder 80 can be moved under the operation of the system controller 400 by using precision linear motors (e.g., piezoelectric actuators) to set the measurement location for the EPCS and LSP sub-systems 100 and 200.

In an example, the prism support structure 46 includes sections that are movable with respect to one another so that the EPCS and LSP coupling prisms 42A and 42B can be moved relative to one another, e.g., axially or the z-direction as shown in FIG. 6C. In an example, the support frame 48 of the prism support structure includes adjacent walls configured so that one wall can slide relative to the other in a controlled manner. In the example of FIG. 6C, the EPCS coupling prism 42A is shown has having moved in the z-direction relative to the LSP coupling prism 42B.

FIG. 6D is similar to FIG. 4C and illustrates an embodiment of the hybrid system 20 wherein the EPCS sub-system 100 and the LSP sub-system 200 share a common coupling prism 42, i.e., the common coupling prism 42 acts as both the EPCS coupling prism 42A and the LSP coupling prism 42B. A single index-matching fluid 5 is also used. The various surfaces of the common coupling prism 42 have dual purposes, e.g., the coupling surface is denoted 45A and 45B because it serves the dual purpose of performing EPCS coupling and LSP coupling. In an example, the band-pass filters 144 and 244 of the EPCS sub-system 100 and the LSP sub-system 200 are used, along with different wavelengths λ_(A) and λ_(B) (e.g., separated in wavelength by at least the bandwidth of one of the band-pass filters 144 and 244) to substantially reduce or eliminate cross-talk between the sub-systems. In the example of the common coupling prism 42, the coupling prism can have an ECSP section PS1 and a LSP section PS2, and further in the example the sections can be separate, i.e., the EPCS light beam 116 and the LSP light beam 216 generally stay in their respective sections, with the exception of small amounts of scattered light.

Reducing Substrate Warp

The CS substrate 10 can be large enough that it can warp to the point where making accurate EPCS and LSP stress measurements becomes problematic. In particular, a warped CS substrate 10 can make it difficult to establishing the EPCS and LSP coupling interfaces INT1 and INT2 needed for making the EPCS and LSP measurements.

With reference again to FIGS. 8A and 8B, the vacuum system 91 is used to reduce or eliminate substrate warp. The PV conduits (PV bars) 90 are in pneumatic communication with the top surface 12 of the CS substrate 10 through the measurement aperture 72 in the support plenum 70, which supports the CS substrate so that the top surface 12 resides substantially at the measurement plane MP. Activation of the vacuum source 92 generates reduced pressure near the coupling prism assembly 40 via the PV conduits 90, resulting in a downward force FD on the CS substrate by the surrounding high pressure, as shown by the two large arrows. The PV conduits 90 enable height control of the CS substrate relative to the top surface 71 of the support plenum 70 (and thus to the measurement plane MP) to within an accuracy of ±5 microns. The use of the vacuum system 91 also reduces vibrations and enables non-contact control of the CS substrate for dynamic processing and inspection without the need to stabilize the CS substrate on a vacuum chuck.

The PV conduits 90 are commercially available and can be configured for reducing warp as shown in FIGS. 8A and 8B. Some of the PV conduits 90 proximate to the coupling prism assembly 40 may need to be omitted to avoid interference with the EPCS and LSP light beams 116 and 216 and various components of the EPCS and LSP sub-systems 100 and 200 that reside immediately below the support plenum 70. In an example, one or more stop members 94 can be used to hold the CS substrate 10 in place on the support plenum 70.

In some cases, it may be desirable that at least one of the EPCS and LSP coupling prisms 42A and 42B be capable of being adjusted independent of the other. In this case, the coupling prism assembly 40 can comprise two separate prism support structures 46, with one or both of them being adjustable. In one example, the EPCS coupling prism 42A is adjustable in the z-direction to optimize the contrast of the TM and TE mode fringes in the mode spectrum. This can be accomplished by using a single-axis micro-positioner operably attached to the prism support structure 46 that holds the EPCS coupling prism in a movable configuration.

Processing EPCS and LSP Measurements

FIG. 9 is a schematic diagram of an example user interface 410 as displayed by the system controller 400 of the hybrid system 20. The user interface 410 includes a EPCS section 412A that shows the mode spectrum 160 generated by the EPCS sub-system 100 and a LSP section 412B that shows the digital LSP image 248D generated by the LSP sub-system 200. The software in the system controller 400 is configured to calculate first stress characteristics of the CS substrate using the EPCS measurements from the EPCS sub-system 100 (i.e., the mode spectrum 160) and calculate second stress characteristics of the CS substrate using the LSP measurements from the LSP sub-system 200 (i.e., the digital LSP image 248D) and then combine the measurements to generate a complete or full stress characterization of the CS substrate.

Processing LSP Measurements

In an example, the system controller 400 is configured (e.g., with software) to process the LSP image 248 to extract the “second” or LSP stress characteristics obtained from the LSP sub-system 200. This includes digitally characterizing the contour of the LSP image 248 using a Gaussian blurred Otsu thresholding is performed as part of the contour detection method to facilitate the calculation of optical retardation vs. depth (OR vs. D).

FIG. 10A is an example representation of the LSP image 248 as shown in the LSP section 412B of the user interface 410. The detection of the LSP image 248 by the digital detector 246 forms a digital LSP image 248D, which can be referred to as a raw LSP image or a raw digital LSP image. The LSP section 412B of the user interface also shows a histogram of the scattered light intensities that constitute the digital LSP image 248D as well as some pertinent statistical measurements. In this example view, the primary beam entrance into the CS substrate is from the lower right to the center of the cross. From the center of the cross to the upper right, the digital camera sees a reflection off the air surface of CS substrate of the side of the beam (see FIG. 11C below) due to total internal reflection. From the center of the cross to the lower left, the direct beam has reflected off the CS substrate air surface and traverses back through the thickness of CS substrate towards the LSP coupling prism. From the center to the upper left, the digital camera views a reflection of the reflected beam.

The digital LSP image 248D is largely comprised of very bright pixels and pixels with little to no exposure. With reference to FIG. 10B, as part of the contour detection method, a Gaussian blur is applied to the original (raw) digital LSP image 248D (left-side image) to reduce any residual noise. The result is a blurred LSP image 248B (right-side image). The Gaussian blurring is applied in a manner that does not obscure the optical retardation information encoded in the intensity variations of the digital LSP image 248D.

With reference now to FIG. 10C, Otsu thresholding is applied to the (Gaussian) blurred LSP image 248B of FIG. 10B to obtain a threshold LSP image 248T. The Otsu thresholding mechanism uses the image histogram (see FIG. 10A) to select an intensity value below which all pixels are set to zero. The bright section in FIG. 10C represents all pixels with intensity above that threshold value.

FIG. 10D shows the next process step, which involves using the threshold LSP image 248T to define an LSP image contour 248C using a binarizing method, such as by the application of the known open-source binarizing algorithm, such as available from open-source image-processing algorithms (e.g., via OpenCV). The example uses an image coordinate system with 0.0 in the upper left and with increasing values to the right (x) direction and down (y) direction. The LSP image contour 248C is comprised of an array of points that can be split into quadrants to find the following five critical points of the cross-shaped image: Upper Left, Upper Right, Lower Left, Lower Right, and Center. The close-up of FIG. 10D shows an example of the lower left point detection, which is obtained by finding the lowest X and highest Y values in that area. The same process is repeated for all four corners, and the center is determined by averaging the corner X and Y values.

FIG. 10E shows the final LSP image contour 248C with a fully defined contour and processed area. In an example, the lower-right leg of the processed “X” LSP image contour 248C (see trapezoidal area) is then used for calculating the LSP stress characteristics. Horizontal lines in the LSP image contour 248C in FIG. 10E are at constant depth. The intensity (e.g. sum, peak, or average) Gaussian blur across the horizontal line from each of the images acquired while the polarization of the light source is being modulated, is used as an input for subsequent analysis to obtain the OR vs. D data.

Thus, the threshold LSP image 248T and the LSP image contour 248C are used to define a “mask” that identifies a portion or portions of the captured or Gaussian-blurred LSP image 248B to be used for calculating the optical retardation OR as a function of depth (D) into the CS substrate 10, as explained above.

CS Substrate Thickness Extraction and Beam Angle Calculation

FIG. 11A is a view of the CS substrate 10. FIG. 11A also shows beam path of a portion of the focused LSP light beam 216F inside the body 11 of the CS substrate 10 after going through the LSP coupling prism 42B (not shown). FIG. 11B is a close-up view showing an edge portion of the CS substrate 10 as the area of interest for calculating the CS substrate thickness TH. FIGS. 11C through 11E are additional views of the path of the focused LSP light beam 216F within the CS substrate. The LSP coupling prism 42B is not shown for ease of illustration.

By looking at the edge of the CS substrate 10 along the direction of propagation of the focused LSP light beam 216F, the thickness of the CS substrate 10 as seen by the digital detector 246 of the LSP detector system 240 can be highlighted, as shown in FIG. 11B. Since the digital detector 246 is looking at the focused LSP light beam 216F through an angled LSP coupling prism 42B (e.g., angled at 45°), the real thickness TH of the CS substrate 10 can be calculated as TH=x/{Cos(45°) where x represents the path length in the plane of the digital detector 246.

Once the thickness TH is calculated, the propagation angle A of the focused LSP light beam 216F within the CS substrate (see FIG. 11E) can be determined by looking at the edge of the CS substrate 10 along the direction of the digital detector 246, and using the schematic diagram of FIG. 11E, determining the propagation angle A using: A=ArcTan(W/TH) where W is the horizontal distance between the center cross C of the LSP image contour 248C and the lower right (LR) critical point of the image contour as obtained from the contour detection method described above. Once the processed area is selected, the digital detector 246 records several LSP images 248 as a function of input polarization. The optical retardation information as a function of depth into the CS substrate is then extracted using techniques known in the art.

Lock-In Detection Method

The lock-in detection method is a signal analysis technique that has proven very adept and fast at retrieving a signal that is obscured with noise. For this method to work, the period of the signal must be known.

The measurement (detector) signal SB from the LSP sub-system 200 has a period that depends on the rate of polarization rotation by the optical compensator 230. When using a rotating half-wave plate 234RH in the optical compensator 230, one full rotation corresponds to four oscillations of the polarization state of the scattered LSP light beam 216S.

The derivation of the lock-in method as applied to the LSP measurement signal SB=s(t) is as follows, where t is time. Consider the LSP measurement signal s(t) to be centered around zero and having an amount of noise (“noise factor”) N. The measurement data D(t) received by the system controller 400 can be represented as: D(t)=s(t)+N The measurement signal s(t) can be generalized in the form s(t)=A cos(ƒt+φ) where φ is the phase value to be extracted and ƒ is the known frequency of the signal. This signal can be “locked” into by multiplying it with a generic test wave of an equal and negative period (and arbitrary phase) W(t)=cos (−ƒ−θ) to yield the following equations:

${{D(t)}*{W(t)}} = {{\left( {{A\cos\left( {{ft} + \varphi} \right)} + N} \right)*\left( {\cos\left( {{- {ft}} - \theta} \right)} \right)} = {{\left( {A{}\cos\left( {{ft} + \varphi} \right)*\cos\left( {{- {ft}} - \theta} \right)} \right) + \left( {N*\cos\left( {{- {ft}} - \theta} \right)} \right)} = {{\frac{1}{2}\left( {{A\cos\left( {\varphi - \theta} \right)} + {A\cos\left( {{2{ft}} + \varphi + \theta} \right)}} \right)} + {N\cos\left( {{- {ft}} - \theta} \right)}}}}$ ${{D(t)}*{W(t)}} = {\left( {{\frac{A}{2}\cos\left( {{2{ft}} + \varphi + \theta} \right)} + {N\cos\left( {{- {ft}} - \theta} \right)}} \right) + {\frac{A}{2}\cos{\left( {\varphi - \theta} \right).}}}$

The first two terms of the equation for D(t)*W(t) immediately above oscillate according to the time variable t. The final term, however, is a constant that can be extracted through strong low-pass filtering of the product wave. Since the average of a wave approaches the offset value of that wave over multiple oscillations, this is achieved by averaging the product wave.

This approximation incurs a slight amount of error if the measurement signal s(t) does not have many oscillations (e.g. less than one full oscillation) or if the signal has a non-integer number of half-cycles. This error can be reduced by taking the average of the signal over only the largest amount of half-cycles in the signal. For example, if the signal has about 3.7 oscillations, then take the average of the signal up to 3.5 cycles.

Once the low-pass filtering is performed using known means, the product D(t)*W(t) is reduced to the constant term [A/2] cos(−θ+φ). Recall that φ is the desired phase value, and θ is the arbitrary phase of the test wave. Hence, if θ is incremented through a series of numbers, the constants resulting from the low-pass filtering of the product wave for each increment will oscillate according to the non-time-varying cosine function [A/2] cos(−θ+φ). This cosine wave has a wave number of −1, an amplitude of A/2, and a phase of φ. Knowing this, a cosine can be fit to these constants (e.g., using least-squares fitting), and the phase φ can be extracted. The amplitude of the signal A can also be extracted.

The lock-in method for signal extraction has proven to be much faster than a regular sine fitting. FIG. 12A is a plot of the average computation time T in milliseconds (ms) needed to extract the phase φ of a noisy signal versus the noise factor N for both the lock-in method (L or black curve) and the sine method (S or gray curve). The data for FIG. 12A were collected over a series of tests. In these tests, random noise was added to a set signal upon which both the sine fitting and the lock-in detection methods were used to extract the phase. At each noise level 100 tests were performed with randomized noise. The lock-in method performed the calculation in approximately half the time that it took the sine fitting method.

FIG. 12B is a plot of the absolute phase difference |Δφ| for versus the noise factor N for the lock-in method (L or black curve) and the sine method (S or gray curve). FIG. 12B shows that both methods retained approximately the same level of accuracy and precision over all the tests

The lock-in method removes the necessity of predicting the sine parameters for fitting. The only fitting performed is that of the cosine wave to the low-pass filtering constants, which is so constrained that it almost never produces a bad fit. If sine fitting is used, however, it has been found that it performs much more accurately when the sine wave that is fitted to the data has a constant period. If the period can be fitted along with the other parameters, the processing time takes longer and the results are often not as accurate.

Noise Reduction in the LSP Measurement

The extraction of second stress characteristics using the LSP measurements from the LSP sub-system 200 consists of two main parts—a data acquisition part and data analysis part. In the data acquisition part of the measurement, the scattered LSP light beam 216S is imaged as a function of the input polarization state of the initial LSP light beam 216 from the LSP light source 212. The imaging is achieved by recording at the digital detector 246 the intensity of scattered light from stress-induced features (e.g., refractive index variations) within the body of the CS substrate 10 due to the IOX process or processes.

The recorded LSP images 248 are processed by the system controller 400 to extract the intensity along the laser beam, which is analyzed against the input polarization to extract the amount of optical retardation between two orthogonal states of the light beam. The stress profile is reconstructed by modeling the observed retardation. As a result, the quality of LSP measured stress profile is fundamentally limited by the noise in the imaging process, which is typically dominated by laser-based noise. One example of such laser-based noise is speckle, which originates from the high degree of coherence of the LSP light source 212, and imperfections in the optical surfaces (roughness, flatness, etc.) and volume properties of the optical elements (impurities, inhomogeneity and heterogeneity of density, etc.).

During propagation of the LSP light beam 216 through the LSP sub-system 200, interactions of the LSP light (laser) beam 216 with system imperfections result in random amplitude and phase variations within the light beam wavefront. When the LSP light beam 216 is coherently imaged by Rayleigh scattering, wavefront distortions lead to a static interference pattern in the image plane that is called a speckle pattern, which is characterized by large intensity variations with high spatial frequencies superimposed on the desired signals. Intensity deviations from the desired signals are considered as noise in an LSP measurement. To reduce the effect of laser speckle, the imaging can be averaged over independent speckle patterns by modulation of polarization, amplitude or phase in the beam wavefront.

In one embodiment, laser-based noise is reduced in the LSP sub-system 200 by passing the initial LSP light (laser) beam 216 through a movable light diffuser 222, which in an example can comprise a holographic diffuser. This “stirs” the light beam rays within the diffusing angle depending on the local structure of the diffuser. To minimize the beam divergence caused by this “ray stirring,” the movable light diffuser 222 is placed in the image plane of a Keplerian telescope configuration, as shown in the example configuration of FIG. 4A. The LSP light beam 216 is first focused onto the movable light diffuser 222 by the first focusing lens 220 and the transmitted light beam is re-collimated by the second focusing lens 224.

Mitigating the divergence of the LSP light beam 216 after undergoing light diffusion provides more efficient (i.e., less aberrated) focused LSP light beam 216F at the CS substrate. Using the movable light diffuser 222, variations the laser-based noise (e.g., speckle pattern) at the digital detector 246 are produced at the rotational velocity v_(D) of the spinning diffuser. The maximal effect of noise averaging is achieved at the v_(D)τ_(C)>1 condition, where τ_(C) is the exposure time of the digital detector 246. This condition also eliminates a potential blinking in the imaging caused by optical transmission variations across the movable light diffuser 222. Implementation of diffuser-based noise reduction improves the measurement of the optical retardation. This is illustrated in FIGS. 13A and 13B, which are plots of optical retardation OR (radians) versus depth D (mm) into the CS substrate. The plot of 13A was obtained without using the noise reduction apparatus and methods described above. The plot of FIG. 13B was obtained by using the noise reduction apparatus and methods described above. The smoothness of the plot of FIG. 13B is a direct result of the application of the noise-reducing apparatus and methods disclosed herein.

Shifting the OR Plot Using the Bend Points and CS Substrate Mid-Plane

Since the location of the top surface 12 of the CS substrate 10 can be difficult to determine from the LSP images 248, the stress profile can be shifted into position based on the general shape of the retardation curve (OR vs D). The OR retardation curve has two bend points where the derivative is zero. An example actual OR vs. D curve is shown in FIG. 14A along with the two bend points BP1 and BP2. The data points are shown as open circles. The two bend points correspond to where the stress profile changes from compression to tension, or vice versa.

If the stress profile is symmetric, then the two bend points BP1 and BP2 should also be symmetric around the mid-plane MP of the CS substrate (see FIG. 1A). Therefore, if the thickness TH of the CS substrate 10 is known and the two bend points BP1 and BP2 of the optical retardation OR curve can be found, then the OR profile can be horizontally shifted into the correct position. This allows for the depth of compression DOC to be more accurately determined because the location of the top surface 12 of the CS substrate 10 is chosen based on the known symmetry and thickness of the CS substrate. FIG. 14B is similar to FIG. 14A but shows the OR curve shifted to the left as compared to FIG. 14A using the plot-shifting (data-shifting) technique described above.

Shifting the OR Plot Using Curve Fitting

An alternate method of extracting the DOC for a symmetric stress profile involves analyzing the shape of the retardation profile, i.e., the OR vs. D curve. If the thickness TH of the CS substrate is known and the relative positions of the bend points BP1 and BP2 can be determined through polynomial fittings, then the depth of compression DOC of the CS substrate can be determined by expression: DOC=[TH−(BP2−BP1)]/2 where BP1 and BP2 are the relative depth positions of the bend points.

Curve Fitting for the OR vs. D Curve

An embodiment of the disclosure is directed to methods obtaining an excellent fit to the data of the OR vs D curve. The methods include employing a combination of linear and quadratic functions to obtain the curve fit. This method is referred to below as the LinQuad method.

FIG. 15A is a plot of OR vs. D data (circles) and shows an example fitted curve FC (solid line) to the OR data using the LinQuad method. The LinQuad method assumes the following model stress function, where a is the stress, x is the depth coordinate into the CS substrate 10 and R is as defined below:

${\sigma(x)} = {\frac{d\sigma}{dC}\left( {{C(x)} - {R*C_{0}}} \right)}$

One can extract the corresponding retardation and fit it to the raw data of interest to recreate the stress profile. Here, C represent the normalized modeling concentration of ions in the CS substrate. Their expressions are as follows.

$\begin{matrix} {{C(x)} = \left\{ \begin{matrix} {C_{0}\left( {1 - \frac{2x}{d_{c} + d_{l}}} \right)} & {{{for}0} \leq x \leq d_{l}} \\ {C_{0}\left( {1 - \frac{2d_{l}}{d_{c} + d_{l}}} \right)\frac{\left( {d_{c} - x} \right)^{2}}{\left( {d_{c} - d_{l}} \right)^{2}}} & {{{for}d_{l}} \leq x \leq d_{c}} \\ {{0{for}x} \geq d_{c}} &  \end{matrix} \right.} &  \end{matrix}$ $\ {{R = {\frac{2}{3}\frac{d_{c}}{t}\frac{1 + \delta + \delta^{2}}{1 + \delta}}},{\delta = \frac{d_{l}}{d_{c}}}}$ where d_(l) is the depth of the linear region, d_(c) is the depth of the curved region, C₀ is a constant multiplier and t is the CS substrate thickness.

An alternate expression is given by:

${\sigma(x)} = {{CT} - {{C(x)}*\frac{d\sigma}{dC}}}$ Here, CT is the central tension of the stress profile, and

$\frac{d\sigma}{dC}$ is a (partially arbitrary) constant of around

$60{\frac{MPa}{{mol}\%}.}$ The true LinQuad function is defined above, where only d_(c), d_(l), C₀ are fit. However, this latest expression for σ(x) allows a fourth parameter—namely the central tension CT—to vary, which can help the function fit the data more closely.

FIG. 15B is a plot of the stress S(x)=σ(x) versus depth D (mm) (or x coordinate) based on the LinQuad fit to the OR vs. D curve of FIG. 14A.

Power-Spike Function

A power-spike function is defined as:

${\sigma(x)} = {{CT_{sp}} + {CT_{p}} - {C{T_{p}\left( {p + 1} \right)}*{❘\frac{2\left( {x - {mid}} \right)}{t}❘}^{p}} - {\left( {{CS}_{sp} - {p*CT_{p}}} \right)*\left( {2 - {{ERF}\left( \frac{x}{L_{{eff},{sp}}} \right)} - {{ERF}\left( \frac{t - x}{L_{{eff},{sp}}} \right)}} \right)}}$ ${CT}_{sp} = {2*\left( {{CS}_{sp} - {p*{CT}_{p}}} \right)*\frac{L_{{eff},{sp}}}{t\sqrt{\pi}}}$ ${CT}_{sp} = {2*\left( {{CS}_{sp} - {p*{CT}_{p}}} \right)*\frac{L_{{eff},{sp}}}{t\sqrt{\pi}}}$ $L_{effsp} = \frac{{DOL}_{sp}}{{1.3}829}$ where CT_(sp) is the central tension of the spike in the spike region R1, mid is half of the thickness TH, CS_(sp) is the compressive stress of the spike, and DOL_(sp) is the depth of layer for the spike. The parameter L_(eff) is an effective length (depth) of the spike region R1. This function is a stitching of a power profile with two error function spikes at the ends. The CS_(sp) and DOL_(sp) values are specific to each glass type and are entered as a constant. The only parameters that need to be fitted are the power of the function p and the peak central tension CT_(p).

FIG. 16A is a OR vs. D plot illustrating an example fitted curve FC using the power-spike function. FIG. 16B is plot of the stress profile S(x) (MPa) versus depth D into the CS substrate 10 based the power-spike function fit to the OR vs. D curve of FIG. 16A.

Removing Systematic Error to Conform with a Symmetric Stress Profile

The stress profile of the CS substrate using the LSP measurement data is obtained by differentiating the OR vs. D curve. As such, a symmetric stress profile will always correspond to an asymmetric OR vs. D curve. However, systematic error from various components in the LSP sub-system 200 can introduce a symmetric component into the OR vs. D retardation data, thereby hindering the accurate extraction of the stress profile. This effect can be mitigated by decomposing the retardation data into symmetric and anti-symmetric components, and only fitting the anti-symmetric portion (i.e., the asymmetric data points).

Given a optical retardation OR in the form of a function ƒ(x), the decomposition can be achieved as follows. ƒ(x)=ƒ_(s)(x)+ƒ_(a)(x) where ƒ_(s) and ƒ_(a) are the symmetric and anti-symmetric components of the retardation ƒ, and expressed by the following equations:

${f_{s}(x)} = \frac{{f(x)} + {f\left( {- x} \right)}}{2}$ ${f_{a}(x)} = \frac{{f(x)} - {f\left( {- x} \right)}}{2}$

FIG. 17A is fit to the OR vs. D plot based on the original OR data while FIG. 17B is a fit to the OR vs. D plot with the symmetric component of the data removed using the technique described above. The fitting error of the fitted curve FC to the measurement data in FIG. 17B is 0.006 while that of FIG. 17A is about 0.46

Adjustable Fitting Regions for Accurate CT and DOC

A single fit to the OR v. D curve may not always be adequate to accurately determine both the central tension CT and the depth of compression DOC. This is because scattering from the LSP coupling prism 42B or the coupling interface INT2 can hinder data collection close to the top surface 12 of the CS substrate 10.

In an example, the fit to the OR vs. D curve is performed using fits to separate regions of the curve respectively associate with the central tension CT and depth of compression DOC and adjust the fitting range of the OR data for accurate CT and DOC extractions.

FIGS. 18A and 18B show an example OR vs. D curve wherein regions around the bend points BP1 and BP2 defined by the data (circles) are fitted to extract the depth of compression DOC. FIG. 18B shows the central linear region between the bend points BP1 and BP2 fitted to extract the central tension CT. In both cases, the range of the OR vs. D data is substantially reduced to that portion of the OR vs. D curve that is relevant to the given stress parameter.

FIGS. 19A through 19D further illustrate the effect of data range selection (shown by vertical dashed lines) on the fitting quality. FIG. 19A is a OR vs. D plot where the full data range is considered and where the fitted curve does not fit the bend points BP1 and BP2 very closely. FIG. 19B is the corresponding plot for FIG. 19A of the stress S(x) versus D (depth) that shows the compressive stress CT and the depth of compression DOC.

FIG. 19C is an OR vs. D plot similar to FIG. 19A except that the data range is reduced to the region between the vertical dashed lines and so omits first and second “end regions” ER1 and ER2 of the measurement data. The fitted curve FC of FIG. 19C closely follows the bend points BP1 and BP2. The corresponding S(x) vs. D plot is shown in FIG. 19C and the values for the compressive stress CT and the depth of compression DOC differ substantially from that of FIG. 19B in which the full range of data was used.

Simultaneous EPCS and LSP Measurement Considerations

One method of achieving good precision for the measurement of the depth of compression (DOC) using the LSP sub-system 200 is to press the CS substrate 10 against a stop surface (e.g., the support plenum 70) to ensure that the top surface 12 of the CS substrate 10 is co-planar with a pre-defined surface that can be assigned a depth of z=0. This pressing can be achieved by either pushing the CS substrate 10 against the stop, or by applying a vacuum such that the ambient atmospheric pressure provides the force to push the top surface 12 of the CS substrate 10 into place at z=0 (see e.g., FIGS. 8A, 8B).

On the other hand, achieving sharp (i.e., high contrast) mode spectrum 160 using the EPCS sub-system 100 to obtain an accurate stress measurement of the near-surface region R1 of the NSWG 18 also usually requires good CS substrate flatness in the EPCS measurement area, which may be also achieved with the use of the vacuum system 91.

Due to the EPCS and LSP measurement areas being at different locations of the CS substrate, applying vacuum at the LSP measurement area can in some case deform the CS substrate at the EPCS measurement area, and result in sub-optimal, or even very poor flatness or significantly deformed surface in the EPCS measurement area. This results in an EPCS mode spectrum 160 that has poor contrast and is “out of focus”. These conditions can lead to decreased accuracy and decreased precision, as well as a failure to measure because the poor contrast can cause the system controller to fail to identify some of the target features of the mode spectrum 160 used to perform the stress calculations.

In an example embodiment, the EPCS detector system 140 of the EPCS sub-system 100 utilizes adaptive focusing to enable proper alignment of the CS substrate 10 on the support plenum 70 for the best (most precise) DOC measurement and near-surface stress measurements using the EPCS sub-system when the CS substrate is aligned for a best LSP measurement for the LSP sub-system 200.

In one embodiment illustrated in FIG. 20 , this is accomplished by making the focusing lens 142 of the EPCS detector system 140 adjustable, e.g., axially movable by mounting the focusing lens on a translation stage 143, which in an example is operably connected to and controlled by the system controller 400. In an example, the translation stage 143 comprises a precision linear actuator, such as a piezoelectric actuator. In another example, the translation stage 143 comprises ball screw actuator. This allows for the focusing lens 142 to be translated along the second optical axis A2 to improve or maximize the contrast of the mode spectrum 160 captured by the EPCS digital detector 150. In an example, the contrast of the mode spectrum 160 is improved to enhance target spectral features, such as the TM and TE fringes 163TM, 163TE and the critical angle transitions 166TM and 166TE.

The position of the axial movable focusing lens 142 can be monitored electronically by the system controller 400 to correct the EPCS sub-system calibration by accounting for the “optical path length” or OPL, e.g., the distance from the focusing lens 142 to the EPCS digital detector 150. In one embodiment, the accounting can be simplified as long as the OPL does not fall outside of a pre-defined acceptable range, so that the original calibration remains accurate. In another embodiment, the calibration is corrected based on the OPL, and the surface stress S(0)=CS and/or the depth of layer DOL is calculated based on the corrected calibration.

In another embodiment, the focusing lens f1 has variable effective focal length which is actively controlled by the system controller 400 to obtain a high-contrast mode spectrum 160 when the specimen is aligned to ensure most precise or accurate measurement of the depth of compression DOC for the LSP sub-system 200. The variable focal length focusing lens 142 can comprise a compound lens (similar to photographic multi-component lenses with more than one optical element), or maybe otherwise comprise an adaptive lens, such as a fluid-filled lens where varying the pressure of the fluid changes the shape of the lens and thus the focal distance. When using variable focal length focusing lens 142, shifting the position of the focusing lens 142 may not be necessary, as changing the focal length can in many cases be adequate to compensate for the deformation of the specimen shape in the EPCS measurement area as a result of aligning the specimen for best measurement in the LSP-measurement area.

In another embodiment, the variation of effective focal length of the focusing lens 142 may be enabled by an adaptive lens surface in the form of a mirror surface that may be combined with a fixed simple lens to produce a net effective focal length that can be varied over a range sufficient to produce a high-contrast mode spectrum 160 even when the CS substrate alignment is optimized for the LSP sub-system 200.

Since the deformation in the CS substrate 10 tends to not be very large, the change in refractive power for the variable focus focusing lens 142 need not be particularly large to compensate. In an example, the focal length of the focusing lens 142 can be changed by up to 15%, or in another example up to 10%.

On the other hand, when the CS substrate 10 has thickness less than 0.6 mm, it may be necessary to change the refractive power by more than 15%, and by as much as 20% or even 25%. Thus, in an example, the adaptive system for changing the focal length of the focusing lens 142 is configured to change the focal length over a focal length range that represents 25% of the average focal length, although in many cases a range of 20%, 15%, or even 10% of the average focal length may be adequate.

Similarly, since for measurements of flat CS substrates, the focusing lens 142 system is focused on infinity, when the focusing lens 142 has fixed focal length and the position of the focusing lens is axially adjusted to produce a high-contrast mode spectrum 160, the range of axial positions that are accessible by the focusing lens would ideally represent about 25% of the focal length of the lens, although in some cases 20%, 15%, or even 10% of the focal length may represent an adequate range of positions.

FIGS. 21A and 21B are schematic diagrams of example embodiments wherein two or more focusing lenses 142 of slightly different focal lengths are mounted on a support member 152 to define a focusing lens assembly 153. The support member 152 is movable to place a select one of the focusing lenses 142 in the optical path (i.e., along the second axis A2) of the reflected EPCS light beam 116R. This allows a user to choose a focal length of the focusing lens 142 from a discrete set of focal lengths. FIG. 21A shows an example where the support member 152 is in the form of a rotatable wheel that is rotatable about a rotation axis AW. FIG. 21B shows an example where the support member 152 is in the form of a linearly translatable support frame. Four example focusing lenses 142 are shown. In general, the focusing lens assembly 153 can support two or more focusing lenses 142.

If the contrast of the features of interest (e.g., the TM and TE mode lines 163TM, 163TE, the TM and TE critical angle transitions 166TM, 166 TE, etc.) in the mode spectrum 160 is deemed adequate, the measurement proceeds as usual. If the contrast of the features of interest is deemed inadequate, then a focusing lens 142 of a different focal length is moved into the optical path of the reflected EPCS light beam 116R and a new mode spectrum 160 is captured by the EPCS digital detector 150 and the contrast analyzed. This process repeats until a mode spectrum 160 of adequate contrast is obtained.

In an example, differences in the focal lengths of the focusing lenses 142 may be set by the total desired range of focal-length coverage and the total number of lenses on the support member. In one example, there are six focusing lenses supported by the support member, with the focusing lenses covering a range that is between 20% and 30% of the average focal length for the entire set of focusing lenses, and the spacing of focal lengths is between 3% and 7% of the average focal length.

In another example, the focal lengths are spaced unevenly, such that the spacing of each pair of neighboring focal lengths is approximately a fixed percentage of the average of the neighboring focal lengths, where that percentage is between 2% and 20%, and more preferably between 3% and 10%.

In another related embodiment, some or all of the focusing lenses 142 comprise Fresnel lenses. In another embodiment, the focusing lenses 142 need not have different focal lengths, but may be positioned on the movable support member in such a way that when the focusing lens of choice is placed in the optical path, its distance from the EPCS digital detector 150 is different than for the other focusing lenses. In this embodiment, obtaining a spectrum having adequate contrast for the features of interest is guaranteed not necessarily by having a complete set of discrete densely spaced custom-chosen focal lengths, but by a set of distances to the digital detector and/or available focal lengths. This can reduce the cost of the EPCS sub-system 100 by utilizing standard off-the-shelf focusing lenses, and positioning each focusing lens to produce a sharp image for a specific range of warp/curvature of the CS substrate 10.

In an example, the system controller 400 can be configured to select one of the focusing lenses 142 based on a measurement of the contrast of the features of interest of the captured mode spectrum 160.

In another embodiment, measurements can be made by using two or three preferred mode spectra 160 having the best contrast among all captured mode spectra, and then a preferred result may be calculated as an average of the two or three preferred mode spectra. In an example, the preferred result may be calculated as a weighted average of the two or three preferred mode spectra. In a related example, the weight for each preferred spectrum may be proportional to the contrast of a feature of interest the preferred mode spectrum.

Using an Independent Stress Measurement for Stress Measurement Calibration

The EPCS sub-system 100 can be very good at obtaining a high-contrast mode spectrum 160 for a CS substrate formed an IOX process using a Li-based glass, e.g., wherein K ions replace Li and/or Na ions from the glass in the near-surface region. This in turn allows for very good measurements of the knee stress CS_(k) by measuring the birefringence based on the relative positions of the TM and TE critical-angle transitions 166TM and 166TE (see FIG. 3B).

On the other hand, the EPCS measurement of the knee stress CS_(k) usually has lower relative precision than the measurement of the surface stress S(0). In particular, the standard deviation of the measurement of the knee stress CS_(k) is usually several % of its average value, whereas the standard deviation of the surface stress S(0) is usually on the order of 1% to 2% of its average value. In addition, the value of the knee stress CS_(k) as obtained simply as a ratio of the birefringence B of the detected critical angle and the stress-optic coefficient (SOC) differs slightly from the value of the knee stress CS_(k) as obtained from a destructive RNF measurement of the stress profile.

When the EPCS measurement of the knee stress CS_(k) is believed to be less accurate than it could be or should be, it can be due to a systematic error in the measurement of the birefringence of the critical angle. This systematic error can be caused by the TM and TE mode lines 163TM and 163TE being too close to the TM and TE critical angle transitions 166TM and 166TE and further by the particular shapes of the TM and TE refractive index profiles.

When making quality-control measurements, such systematic errors are mitigated by calibrating the EPCS-based measurements of the knee stress CS_(k) measurement with corresponding independent reference stress measurement, which may be a destructive measurement on a CS substrate taken from a set of CS substrates formed using the identical process or from the same batch during the same identical process. In an example, this is accomplished by applying a calibration multiplier K_(cal) based on the independent measurement via the relationship: CS_(k)(EPCS,calibrated)=K _(cal)·CS_(k)(independent). In an example, the calibration multiplier K_(cal) can be used as a general calibration factor for the stress profile calculated by the EPCS sub-system 100 via the equation: S(EPCS,calibrated)=K _(cal) ·S(original) where S(orig) is the originally measured (uncalibrated) stress profile S(z).

Tension Zone Stress Profile Extraction

An IOX process used to form a CS substrate 10 forms a compression zone that defines the NSWG 18. This compression zone extends into the substrate and reaches a zero value, which defines the depth of compression DOC. Beyond the DOC, the compression zone ends and a tension zone begins.

If the stress profile in the tension zone can be accurately extracted, it can serve as a powerful tool to help extract a substantially accurate representation of the stress profile in the compression zone. This can be done by exploiting force-balancing of the stress in the entire CS substrate 10 or half of the CS substrate (i.e., so-called “half force balancing”).

In one embodiment, besides the area of the stress profile in the tension zone (which is represented by the depth integral of tensile stress from one depth of compression to the opposite-side depth of compression), a reliable value of the slope of the stress profile at a depth of reliable slope extraction is also obtained from the LSP-based measurement.

In an example, the depth of reliable slope extraction may be the depth of compression DOC. In the compressive-stress region, a surface compressive stress is determined by the EPCS method. In some cases, a portion of the compressive-stress profile is also extracted from EPCS method using prior-art techniques such as IWKB, or linear-profile, erfc-shaped profile, exponential-profile, or a LinQuad profile approximation when there are not enough guided modes for reliable IWKB extraction. The EPCS-based method then provides a target point of connection, either at the surface with a surface stress value S(0), or at a deeper connection point (for example, the knee depth z_(k); see FIG. 1B), up to which the surface portion of the stress profile S(z) can be extracted from EPCS measurement. In the latter case, the knee stress CS_(k) may not be specified with high accuracy due to limitations of the EPCS measurement.

Nonetheless, this value of the knee stress CS_(k) can provide an adequate starting point for pursuing extraction of the stress profile in the compression zone (e.g., substantially zones R1 and R2 in FIG. 1B) by iterative improvement. In a first iteration, the near-surface connection point with surface stress value S(0) may be connected with a deep connection point (e.g., the knee stress CS_(k) or the depth of compression DOC) with a reliably extracted stress slope using a second-order polynomial. This determines a first approximation of the stress profile in the compression zone, having a first portion obtained from EPCS up to the first connection point (say, at the knee depth z_(k)) and a second portion obtained via polynomial interpolation between the two connection points, where at the second connection point not only the surface stress S(0) matched, but also the stress profile slope.

In a particular example, the second connection point can be the depth of compression DOC, but it need not be. The first approximation of the stress profile S(z) is integrated. If the stress profile is asymmetric, EPCS measurements may be performed on both sides of the specimen, and first approximations of the stress profile obtained as above for each side. If the stress profile S(z) is symmetric by design and implementation, then it may be assumed that the back side of the specimen has the same stress profile in the back-side compression region as the front-side compression region.

The first approximation of the stress profile from both front-side and back-side compression zones is integrated with respect to depth over the respective compression zones, and compared to the depth integral of tension over the tension zone. If the difference is larger in absolute value than a pre-defined acceptable limit, a corrective step is performed to reduce the difference. In an example, the pre-defined acceptable limit is 5% of the tension-zone stress area, but progressively better acceptable limits include 3%, 2%, 1%, and 0.5%.

The acceptable limit may be determined based on an estimate of the degree of accuracy of extraction of the tension-zone stress profile. In one embodiment, several first approximations for the stress profile are obtained by different methods, all of which match the knee stress CS_(k) at the first connection point, and the stress value and stress slope at the second connection point, say the depth of compress DOC. Different types of first approximations may include second, third, and 4^(th) order polynomials, an exponential profile, an erfc-shaped profile, a Gaussian profile, and a Lorentzian profile. Then, for each of these first approximations, the difference is found between the stress area in the first-approximation compression zone, and the stress area in the tension zone extracted using LSP-based measurement. Then a linear combination of these first-approximation stress profiles is found, such that the stress area of the linear-combination stress profile equals the tension-zone stress area.

In another embodiment, the limited accuracy of the EPCS-based measurement of the knee stress CS_(k) is taken into account by allowing a range of the knee stress CS_(k) to be targeted around the initial EPCS-based estimate of the knee stress CS_(k). In a first approximation of the compressive-stress portion of the stress profile, the connection is made with the EPCS-based initial value of the knee stress CS_(k), using a preferred target shape function for the interpolated region of the compression zone. In an example, the preferred target shape is a second-order polynomial.

After each iteration, the stress area of the compressive-stress profile from the two combined compression zones (one on each side of the specimen) is subtracted from the stress area of the tension zone. If the difference is larger in absolute value than a target pre-defined acceptable limit, then the target value of the knee stress CS_(k) may be changed within a pre-defined acceptable range for the knee CS_(k) as determined in accordance with an estimated precision of the knee stress measurement available from the EPCS-based method.

In an example, an estimated knee stress precision is about 10 MPa, though in some cases it is better at 7 MPa or 5 MPa or 3 MPa. When no surface spike is present, and no guided modes are available, then the same technique may be used to connect to a target surface stress S(0) that is allowed to vary in a range determined by the precision of the surface stress measurement.

In an example, the range for acceptable values for the target surface stress S(0) or knee stress CS_(k) may be up to 6 standard deviations wide, e.g., 3 standard deviations on either side of the measured value of surface stress or the knee stress. In one embodiment, the target surface value S(0) need not be varied iteratively, but may be determined by algebraic calculation, utilizing the measured difference in area between the first-approximation stress profile and the tension-zone stress profile and the preferred functional form chosen for the interpolated portion of the compressive-stress region.

Enhanced EPCS Sub-System

FIG. 22A is similar to FIG. 3A and illustrates an example of an enhanced EPCS sub-system 100. The enhanced EPCS sub-system 100 places the EPCS light source 112 at a remote location relative to the EPCS coupling prism 42A. The remote EPCS light source 112 is multiwavelength and can include one or more light source elements 123 (e.g., 123 a, 123 b, 123 c, . . . ) that respectively emit light of different wavelengths λ_(a), λ_(b), λ_(c), . . . . The multiwavelength EPCS light source 112 can also include a single light source element 123 that emits broadband light, wherein the different wavelengths fall within or at different locations within the broadband light spectrum.

The EPCS light source 112 is optically connected to a focusing optical system 121 by a light guide 130 having an input end 131, an output end 132 and an axial length AL (see the close-up inset of a stretched out light guide 130 at the bottom of the Figure). In an example, the focusing optical system 121 can comprise one or more lens elements. In an example, the focusing optical system 121 can be color-corrected, i.e., be designed to image multiple wavelengths without substantial chromatic aberrations. This can be accomplished using achromatic lenses and optical coatings designed for the wavelengths or wavelength range employed.

In an example, the light guide 130 can comprise a liquid-filled light guide, such as is available from ThorLabs, Inc., Newton, New Jersey, as part number LLG5-4T. In another example, the light guide 130 can comprise an optical fiber bundle or a solid light pipe. An example light guide 130 has an axial length AL of at least several inches and preferably 8″ or longer, with the precise length determined by the location of the remote EPCS light source 112.

The EPCS light beam (hereinafter, light or light beam) 116 from the remote EPCS light source 112 is coupled into the input end 131 of the light guide 130 travels therein as guided light 116G. The light coupling can be facilitated by using one or more lens elements 135 (see FIG. 23 ). The guided light 116G travels to the output end 132 of the light guide 130 and is emitted as the initial EPCS light beam 116. This initial EPCS light beam 116 emitted from the output end 132 of the light guide 130 is divergent. This divergent light is received by the focusing optical system 121, which forms there from the focused EPCS light beam 116F, which is directed to the EPCS coupling prism 42A. The rest of the example EPCS sub-system 100 of FIG. 22A is as described above in connection with FIG. 3A. The light guide 130, the lens elements 135 and other optional optical components between the light source and the light guide 130 (e.g., the diffuser 137, introduced and discussed below) constitute a light guide assembly that transfers EPCS light beam 116 (either sequentially filtered or multiwavelength) from the EPCS light source 112 to the focusing optical system 121 that resides proximate the EPCS coupling prism 42A.

FIG. 22B is similar to FIG. 22A and illustrates an embodiment wherein the split TM-TE polarizer 148 is replaced by a switchable TM-TE polarizer 148S. The switchable TM-TE polarizer 148S is controlled by a polarizer controller 149, which is operably connected to the system controller 400. In this configuration, the EPCS digital detector 150 sequentially detects a TM mode spectrum 161TM and then a TE mode spectrum 161TE for each wavelength rather than detecting a combined TM-TE mode spectrum 160 for each wavelength (see FIG. 3B) when using the split TM-TE polarizer 148 of FIG. 3A. This approach increases the number of detector pixels available for the mode spectrum images 161TM and 161TE and obviates some resolution/contrast issues that occur at the boundary between the combined TM-TE mode spectrum 160 when using the split TM-TE polarizer 148. An example switchable TM-TE polarizer 148S comprises a magnetically charged polarizing crystal as is known in the art of polarization switching. The analysis of the sequentially captured mode spectrum images 161TM and 161TE is the same as that for simultaneously captured mode spectrum images using the fixed TM-TE polarizer 148.

FIG. 23 is a schematic diagram of an example embodiment of the remote EPCS light source 112. The example remote EPCS light source 112 comprises multiple light source elements 123, with three light source elements 123 a, 123 b and 123 c shown by way of example. The example light source elements 123 a, 123 b and 123 c respectively emit light beams 116 a, 116 b and 116 c having the respective wavelengths λ_(a), λ_(b) and λ_(c). The light beams 116 a, 116 b and 116 c are directed to travel along a common light-source axis AS using wavelength-selective elements 125 a and 125 b, which in example are dichroic mirrors respectively configured to transmit light of one wavelength band while reflecting light of another wavelength band. In an example, the light beams 116 a, 116 b, 116 c combine to form a single multiwavelength EPCS light beam 116.

In an example, the light source elements 123 are operably connected to and controlled by the system controller 400 or alternatively by a light-source-element (LSE) controller 405 operably connected to the system controller 400. The light source elements 123 can be activated simultaneously or sequentially as discussed in greater detail below. The light source elements 123 need not be narrow band and in an example can collectively produce light that is relatively broad band to provide a relatively wide range of measurement wavelengths, such as over the range from near UV to near IR.

With continuing reference to FIG. 23 , the example light source system further includes an optical filter apparatus 500, such as disclosed in U.S. Provisional Patent Application Ser. No. 63/002,468, filed on Mar. 31, 2020, and which is incorporated by reference herein in its entirety. FIG. 24A is a front-on view of a portion of the optical filter apparatus 500 referred to as the filter wheel 530. The filter wheel 530 comprises a support member 510 that operably supports in two or more apertures 516 that respectively contain two or more optical filter assemblies 600, which can be denoted 600 a, 600 b, . . . 600 m for an integer number m of optical filter assemblies. The different optical filter assemblies 600 a, 600 b, 600 c, 600 d, . . . 600 m each have a filter axis AF and are configured to perform narrow-band optical filtering of the EPCS light beam 116 at respective wavelengths λ_(a), λ_(b), λ_(c), λ_(d), . . . λ_(m) having respective relatively narrow bandwidths δλ_(a), δλ₄, δλ_(c), δλ_(d), . . . δλ_(m), such as 2 nm for example. The optical filter apparatus 500 is thus configured to perform narrow-band optical filtering using the optical filter assemblies 600 so that relatively wide band light (e.g., greater than 3 nm or greater than 5 nm, or greater than 10 nm or greater than 20 nm, etc., and including very broadband light with a bandwidth of 100 nm or many hundreds of nm) that includes a given wavelength can be narrow-band filtered around that wavelength. In the discussion below, light that has passes through one of the optical filter assemblies is referred to as filtered light. The filtered light is narrow band and has the center wavelength defined by the filter through which it has passed.

FIG. 24A shows an example filter wheel 530 wherein the support member 510 supports four different optical filter assemblies 600 (600 a, 600 b, 600 c and 600 d) having respective narrow-band filter (center) wavelengths of λ_(a), λ_(b), λ_(c) and λ_(d). The example support member 510 of FIG. 24A has a circular disc-shaped body with a central axis AW, a central section 512 and an outer section 514, with the optical filter assemblies being supported in the outer section, and in an example evenly distributed thereover. The support member 510 also has an outer perimeter 523, a front side 522 and a back side (not shown). The central axis AW runs through the central section 512 of the circular disc-shaped body as shown. The combination of the support member 510 and optical filter apparatus 500 constitute the filter wheel 530.

With reference again to FIG. 23 , a drive system 540 is mechanically connected to the support member 510 and is configured to cause the movement of the support member. An example drive system comprises a drive shaft 544 having one of its ends attached to the central section 512 of the support member 510 while its other end is attached to a drive motor 550. The drive shaft 544 is disposed co-axially with the support member axis AW. The drive motor 550 is electrically connected to the system controller 400, which is configured (e.g., using control software) to control the operation of the drive motor 550 using for example a motor control signal while also receiving a data signal that includes information about the motor operation, such as the rotation rate, the relative rotational position of the filter wheel 530, etc.

The drive system 540 causes the filter wheel 530 to rotate about a rotation axis AR that is coaxial with the support member axis AW. The filter wheel 530 is in turn disposed such that the optical filter apparatus 500 sequentially intersect the light source axis AS so that the EPCS light beam 116 is sequentially filtered to form a sequentially filtered EPCS light beam 116 (i.e., one that includes a sequence of filtered light beams 116 a, 116 b, . . . 116 m), as shown downstream of the optical filter assembly in FIG. 23 and in the schematic diagram of FIG. 24B. FIG. 24B shows two exemplary sequentially filtered EPCS light beams 116, with the top example formed using four filter wavelengths (116 a, 116 b, 116 c, and 116 d) and the bottom example formed using two filter wavelengths (116 a and 116 b). The sequentially filtered EPCS light beam 116 then enters the input end 131 of the light guide 130. As noted above, this can be facilitated by using one or more lens elements 135 operably disposed along the light source axis AS between the filter wheel 530 and the input end 131 of the light guide 130.

The timing of the activation of the light source elements 123 and the position of the optical filter assemblies 600 is controlled by the system controller 400 so that the appropriate portion of the sequentially filtered EPCS light beam 116 is filtered. Alternatively, all the one or more light source elements 123 can be activated at once to form a broad-band EPCS light beam 116, with the optical filter apparatus 500 used to sequentially define the filtered light beams 116 a, 116 b, . . . 116 m that enter the light guide 130 and are eventually directed to the prism assembly 40 as the filtered and focused EPCS light beam 116F (see FIG. 22A)

In an example, the optical filter apparatus 500 is focus corrected to compensate for different focuses of the different wavelength portions of the sequentially filtered EPCS light beam 116, with the different focuses being due to the different wavelengths. The focus correction can be accomplished by the use of correction lenses 124 a, 124 b, 124 c, . . . configured (e.g., axially positioned, having different optical powers, etc.) so that the a given one of the sequentially filtered light beams 116 a, 116 b, . . . 116 m is efficiently coupled into the input end 131 of the light guide 130 for each wavelength of light used. In another example, the focus correction can be employed by each optical filter assembly 600 having a filter 620 and a correcting member 630 configured for focus correction at the given filter wavelength, as shown in the close-up inset in FIG. 23 and as described in detail in the aforementioned U.S. Provisional Patent Application No. 63/002,468.

It is noted that the optical filter apparatus 500 can also be deployed within the EPCS detector system 140 to accomplish the same goal of providing narrow-band measurement light of different wavelengths so that the mode spectrum of the given CS substrate 10 can be measured at each of a number of different wavelengths in order to improve measurement accuracy of a stress-related characteristic of the CS substrate.

The illumination provided to the EPCS coupling prism 42A by the EPCS light source system 110 can be determined by overall system timing and the wavelengths of interest employed. In one example, the EPCS light source 112 can include a single broadband light source element 123, such as a Halogen bulb, and, incandescent bulb, a xenon bulb or a white-light LED or laser diode, and rely on the optical filter assemblies 600 for choosing the wavelengths and band passes. In another example, multiple light source elements 123 that emit different wavelengths can be used to produce light at the wavelengths of choice, such as in the manner described above. In the illustrated embodiment of FIG. 23 that utilizes three light source elements 123 (123 a, 123 b, 123 c) and two wavelength-selective elements 125, two of the light source elements can respectively emit short and long wavelengths over relatively narrow bands while and the third has relatively broad band emission at an in-between wavelength. For example, the short wavelength could be 365 nm, the long wavelength could be 780 nm, and the broader band wavelength might be a human eye white light device comprising a dominant blue (450 nm) LED that includes wavelengths of 450 nm, 510 nm and 640 nm. These three light source elements 123 can be activated simultaneously and continuously with a simple continuous power supply configuration, relying on the bandpass filter configuration of the optical filter apparatus 500 to only pass the desired (narrow-band) wavelength in a sequential manner. Adding control of the beam intensity (e.g., using a variable optical attenuator) permits optimizing the LED output at select wavelengths. This can be useful to compensate for any wavelength sensitivity of the EPCS digital detector 150.

In an example where one or more of the light source elements 123 comprise LEDs, the LSE controller 405 or the system controller 400 can be configured to provide pulsed operation to extend the LED lifetime and reduce the need for heat dissipation, thereby eliminating the need for heat sinks and resulting in a much more compact EPCS light source 112. The pulse control system for the LEDs can include current and/or duration control to permit better matching the light available to the band pass filter transmission and detector sensitivity at the given wavelength. For example, when a “white” LED is being used to provide several different measurement wavelengths, the digital detector exposure time can remain consistent with the available light being controlled by the LED driver. Pulsing the LEDs during the detector exposure time can also permit low duty cycle overdrive of the LED to obtain higher intensity.

The focused configuration of the EPCS sub-system 100 (i.e., wherein the focusing optical system 121 forms the focused EPCS light beam 116F) facilitates accurate optical alignment with little loss of light during the EPCS measurement of the CS substrate 10. The range of measurement angles relative to the normal of first coupling interface INT1 (see FIGS. 22B and 22C) is related to the angle of total internal reflection. When the CS substrate 10 has an index gradient that generally decreases from the top surface 12 into the body 11 of the CS substrate, the range of angles needed to probe the TM and TE mode lines (fringes) 163TM and 163TE (see FIG. 3B) increases but is still within a limited range. By optimizing the light source and detector view angles (i.e., more specifically, enlarging the range of included angles in the filtered and focused EPCS light beam 116F and thus in the filtered and reflected EPCS light beam 116R), the light collection efficiency is increased, resulting in better images of the mode spectrum 160 for the different wavelengths used.

FIG. 22C is a close-up simplified view of the EPCS sub-system 100 in the vicinity of the EPCS coupling prism 42A and corresponding portions of the EPCS light source system 110 and EPCS detector system 140 illustrating an example illumination configuration. In particular, a Kohler configuration is employed wherein the output end 132 of the light guide 130 is imaged through the EPCS coupling prism 42A by the focusing optical system 121 onto an entrance pupil EP of the EPCS detector system 140. In an example, the entrance pupil EP resides at the focusing lens 142 of the EPCS detector system 140 as shown. This configuration ensures that the full radiance of the sequentially filtered EPCS light beam 116 emitted from the output end 132 of the light guide 130 is preserved in the EPCS detector system 140. It also ensures equal illumination of all angles through the EPCS coupling prism 42A, which results in substantially even intensity of the TM and TE mode lines (fringes) 163TM and 163TE captured by the EPCS digital detector 150.

An alternative configuration has the output of multiple light source elements 123 (such as a 2d or 3d cluster) focused at the input end 131 of the light guide 130 to facilitate efficiently coupling the sequentially filtered EPCS light beam 116 into the light guide. In an example shown in FIG. 23 , a diffuser 137 (such as limited-angle diffuser) can be employed at or near the input end 131 of the light guide 130 to improve the light distribution of the sequentially filtered EPCS light beam 116 entering the light guide and simplify the combiner by eliminating the use of dichroic mirrors.

The measurement methods using the enhanced EPCS sub-system 100 are relatively fast and are based on controlling the timing relationships between activation of the light source elements 123, the rotation (azimuthal) position of the filter wheel 530 of the optical filter apparatus 500 and the exposure time (image capture time) of the EPCS digital detector 150. In an example configuration of enhanced EPCS sub-system 100 used in experiments, the motorized filter wheel 530 was controlled using a ThorLabs FW103H high-current BSC201 Controller having 8-32 taps. This provided the optical filter apparatus 500 with a (stepped) filter switching speed of between about 55 milliseconds (ms) to 60 ms. In an example experimental configuration, the optical filter assemblies 600 had a diameter of dimensions up to 1″ with a 6.35 mm thickness. This filter switching speed is still slower than the read out time of the EPCS digital detector 150, which in an example can read 100 frames per second, so that the filter switching speed was the limiting factor in the measurement speed of the example experimental configuration.

An alternative to the fast changing (stepping) filter wheel configuration is a continuously rotating filter wheel 530. The fast change configuration requires a high current drive system 540 to start and stop the filter wheel 530 with adequate precision. A drive system 540 configured to provide constant rotation of the filter wheel 530 uses relatively little ongoing current to keep the filter wheel spinning while inertia from the mass of the filter wheel and optical filter assemblies 600 keep the motion sufficiently constant so that a simple encoder, or simpler reference index sensor(s) can be used to indicate position and even provide a trigger to the EPCS digital detector 150. The trigger can be used to activate the illumination from the light source elements 123 if they are not on continuously. In one example, the exposure time occurs within the duration when the majority (and in on further example, a maximum amount) of the measurement light passes through the given optical filter assembly 600. In another example, the exposure time occurs only when there is light passing through a given optical filter assembly 600 or when there is a minimum amount of light passing through a given optical filter assembly (e.g., more than 10% of the maximum amount).

The design of the support member 510 that forms the filter wheel 530 can include relatively large opaque regions between adjacent optical filter assemblies 600. This can allow the opaque regions to act like focal plane shutters and maximize the exposure duration to the full dimensions of the given optical filter assembly 600 across the given mode spectrum image. The exposure can start while the light is blocked by the opaque region, i.e., before the optical filter apparatus 500 reaches the light source axis AS and the light path of the light. The exposure continues as the optical filter assembly 600 enters and crosses the light source axis AS and then moves away. The exposure stops when the optical filter assembly completely exits the light path of the light and the next opaque region finally reaches the light source axis.

FIGS. 25A and 25B are a close-up views of a portion of an example EPCS detector system 140 illustrating two exemplary configurations of the enhanced EPCS sub-system 100 disclosed herein. The examples of the EPCS detector system 140 employ multiple EPCS digital detectors 150, with the three example EPCS digital detectors denoted 150 a, 150 b and 150 c, which are each operably connected to the system controller 400. The three example EPCS digital detectors 150 a, 150 b and 150 c are spatially separated using beam splitters 180, which in the example of FIG. 25A can be conventional beam splitting elements. In one example, the EPCS digital detectors 150 a, 150 b and 150 c are used to detect respective wavelengths λ_(a), λ_(b) and λ_(c). This can be accomplished by placing respective narrow-band optical filters 144 a, 144 b and 144 c in front of each of the EPCS digital detectors 150 a, 150 b and 150 c. This configuration allows for simultaneous detection of mode spectrum images and obviates the need for the optical filter apparatus 500 and its high-speed filter wheel configuration, but requires multiple EPCS digital detectors 150 and data synchronization to handle simultaneous mode spectrum measurements. The relative axial positions of the EPCS digital detectors 150 a, 150 b and 150 c along the respective optical paths can be set to compensate for any focusing differences due to the different wavelengths of light employed. Likewise, additional focusing lenses 142 a, 142 b and 142 c can be employed to obtain proper focus at the respective EPCS digital detectors 150 a, 150 b and 150 c.

In the configuration shown in FIG. 25B, the beam splitters are wavelength-selective elements 125 such as dichroic mirrors, so that the select wavelength is directed to the corresponding EPCS digital detector. This obviates the need for individual narrow-band optical filters in front of the EPCS digital detectors 150.

FIG. 25C is similar to FIG. 25A except that each single optical filter is replaced by an optical filter apparatus 500 that includes multiple optical filters arranged on a filter wheel 530 as described above.

The cost of using the light source system configuration employing a single optical filter apparatus 500 having a high-speed filter wheel 530 as shown in FIG. 23 could exceed that of using multiple EPCS digital detectors 150 as shown in FIGS. 25A and 25B so that in some embodiments where cost is a factor, the use of multiple EPCS digital detectors may be preferred. The use of multiple EPCS digital detectors 150 has the advantage that the detectors need not be operated at a high frame rate for overall system speed since the speed comes from the simultaneous acquisition of the mode spectrum images using the multiple EPCS digital detectors 150. In this case, the exposure time drives the acquisition time, with the exposure time driven mainly by the intensity of the light source elements 123. The use of relatively high-power light source elements 123 can be used to drive down the exposure time.

Processing mode spectra 160 captured at different wavelengths allows for a more accurate analysis of the stress-related properties of the CS substrate 10. In some cases, the TM mode lines 163TM of a given mode spectrum 160 taken at one wavelength are sharper (i.e., higher contrast) than the TE mode lines 163TE and vice versa. So, use of multiple mode spectra 160 captured at different wavelengths allows for the best (highest contrast) TM and TE modes lines to be analyzed. A high contrast mode spectrum 160 allows for the positions of the TM and TE mode lines 163TM and 163TE to be accurately determined. Further, the positions of TM and TE mode lines 163TM and 163TE can be interpolated through points over the range of measurement wavelengths to better determine the true positions of both real and virtual mode lines to extend the analysis.

The use of a remote EPCS light source 112 external to the main housing of the hybrid system 20 preserves space within the hybrid system, removes a heat source and provides access to service the components of the light source.

Enhanced LSP Sub-System

FIG. 26 is similar to FIG. 4A and illustrates an example of an enhanced LSP sub-system 200. The LSP light source system 210 of the enhanced LSP sub-system 200 now includes the following components operably disposed in order along the third axis A3 between the LSP light source 212 and the first focusing lens: a shutter system 280, a rotating half-wave plate 234RH, and a fixed polarizer (with a fixed polarization direction) 234F. The shutter system 280 can comprise for example a rotating shutter driven by a drive motor. In an example, the LSP light source 212 comprises a relatively high-power laser diode 213 with a light output (LSP light beam 216) centered at a select wavelength, such as 405 nm. In various examples, the laser diode 213 has an output power of at least 1 milliwatt (mW) or at least 10 mW or at least 20 mW or at least 30 mW or at least 40 mW or at least 50 mW.

The optical compensator 230 now includes a spectrometer 260 operably disposed along a spectrometer axis AS defined by the polarizing beam splitter (PBS) 232. The optical compensator 230 also includes a fixed half-wave plate 234H, and a variable polarizer 234V arranged along the third axis A3 and downstream of the PBS 232. The variable polarizer 234V is driven by a polarization controller 237. A movable focusing lens 236 resides downstream of the optical compensator 230. In an example, the movable focusing lens 236 is moved by a linear motor 272 and is mechanically attached thereto by a lens mount 270. In an example, the lens mount 270 can comprise a lens tube that is axially movable (slidable) within a support tube (not shown). In an example, the linear motor 272 comprises a linear actuator configured to provide precise linear movements. In an example, the variable polarizer 234V comprises a liquid crystal variable retarder (LCVR). In an example, a temperature controller 235 is in operable communication with the variable polarizer 234V to provide temperature stabilization to avoid changes in polarization with temperature.

Use of a relatively high-power laser diode in the LSP light source 212 facilitates making stress-related measurements for CS substrates 10 made of a glass material, which has an amorphous structure that does not scatter as much as the crystalline structure of a glass ceramic material. The PBS 232 is optimized for performance at the select wavelength (e.g., 405 nm), which optimizes the amount of light directed towards the CS substrate 10 under test. The combination of the half-wave plate 234H and the PBS 232 allows for automatic light intensity control at the CS substrate 10, making it possible to alternate between measuring glass and glass-ceramic CS substrates.

The spectrometer 260 is disposed to receive a portion of the LSP light beam 216 and spectrally process the LSP light beam. The spectrometer 260 serves two main functions. A first function is to monitor in real time the wavelength of the LSP light beam 216, thereby allowing for wavelength calibration. A second function is to monitor changes in the output power of the laser diode. Thus, the term “spectrally process” can include performing at least one or both of the aforementioned first and second functions.

In an example, the temperature controller 235 allows the LCVR of the variable polarizer 234V to be operated at a slightly elevated temperature relative to an ambient temperature such as room temperate (e.g., between about 35° C. and 40° C.), thereby reducing the effect of room temperature fluctuations on polarization and improving measurement stability and cycle time. In an example, the spectrometer 260 can be a commercially available spectrometer or can be one fabricated from standard components, such as diffraction gratings, a series of narrow-band notched filters, etc., as is known in the art.

The high-power laser diode 213 having about 50 mW of output power may be above the required power needed to obtain enough scattering intensity for certain types of CS substrates 10. However, the exact amount of the LSP light beam 216 entering the CS substrate 10 can be precisely controlled with the rotating half-wave plate 234RH in conjunction with the PBS 232.

FIG. 27A is a schematic diagram that shows an example of how to perform intensity control of the laser diode 213. A linearly polarized LSP light beam 216 from the laser diode 213 passes through the rotating half-wave plate 234RH and is incident upon the PBS 232, which splits the LSP light beam into two beams directed to and detected by the photodetectors PD1 and PD2, which in turn are operably connected to the system controller 400. Note that in practice portions of the LSP light beam 216 can be directed to photodetectors PD1 and PD2 incorporated into the enhanced LSP sub-system 200 using additional beam splitters (not shown). For example, one beam splitter can be placed along the axis AS upstream of the spectrometer 260 while another beam splitter can be placed just downstream of the PBS 232.

FIG. 27B is a plot of the normalized optical power OP versus the angle θ (°) of the rotating half-wave plate 234RH for the transmitted (T) and reflected (R) portions of the LSP light beam 216 formed by the PBS 232 for an example experimental configuration using a laser diode 213. It is seen that the measured powers at the photodetectors PD1 and PD2 vary when the rotating half-wave plate 234RH is rotated. The mirror image signature of the T and R plots is expected, as the rotating half-wave plate effectively rotates the polarization of the light prior to entering the PBS 232. The amount of vertically and horizontally polarized intensities depends on the polarization orientation of LSP light beam 216 from the laser diode.

With reference again to FIG. 26 , the movable focusing lens 236 provides a stable solution for focusing the LSP light beam 216 onto the CS substrate 10 being analyzed with micrometer resolution. In addition, computer-control (via system controller 400) ensures repeatability across different hybrid systems 20, as the position of the focusing lens 236 can be made the same.

The absolute position of the focusing lens 236 can affect CT (central tension) measurements of the CS substrate 10. FIG. 28 is a plot of CT (MPa) versus focusing lens position LP (mm) (measured relative to a reference position) based on measurements performed on an example CS substrate 10. The measurements show about 5 MPa variation in the measured CT across a 2 mm range of motion for the focusing lens 236. The proper placement of the focusing lens 236 ensures accurate and repeatable stress-related measurements.

As noted above, an example variable polarizer 234V comprises a LCVR driven by the polarization controller 237. During a typical measurement using the LSP sub-system 200, the LCVR is excited by a series of voltages provided by the polarization controller 237, with a delay of about 200-milliseconds between voltages. This time delay allows for device response time and image capture and accounts for most of the measurement cycle time.

Although the LCVR operates well at room temperature, its response time can improve substantially (e.g., by up to 3-fold) with increasing temperature, which reduces the response time by reducing the viscosity of the liquid crystal material in the LCVR. In an example, the LCVR is excited with an amplitude-modulated (AM) signal and its response time is determined using an oscilloscope. A decreasing trend of the LCVR settling time with temperature supports the feasibility of measurement cycle time reduction. In addition, as noted above, operating at an elevated temperature relative to the room (ambient) temperature (e.g., >30° C.) can reduce or eliminate the effect of temperature fluctuations on polarization, thereby resulting in higher stability and repeatability across multiple stand-alone LSP sub-systems 200.

An example enhancement to the LSP sub-system 200 includes a calibration process that includes camera angle calibration and camera tilt compensation. FIG. 29 is similar to FIG. 11B but with different labels for the pertinent parameters. The camera angle is denoted β and is measured relative to a horizontal axis while t_(r) is the real CS substrate thickness (which can be measured with a caliper, for example) and t_(i) is the perceived CS substrate thickness in the image plane (i.e., at the digital detector 246). The camera viewing plane is denoted CVP and the viewing direction is denoted z (z-axis). The camera angle β is related to the parameters t_(i) and t_(r) via the relationship Cos β=t_(i)/t_(r).

A camera tilt angle θ measures an amount of camera rotation around the z-axis. A deviation in the camera tilt angle θ can change the way the image plane thickness is calculated. As shown in FIG. 29 , the calculation for the CS substrate thickness t_(r) is no longer the vertical distance between the entrance and exit points, but requires more calculations using trigonometry to account for the deviation angle θ.

An aspect of the disclosure includes an enhanced beam centering method that involves two main steps, namely an improved beam-center step and an improved center-point detection step for an X-shaped LSP image 248 (including one of the processed images as described above) as formed by the detected scattered LSP light beam 216S. (see FIG. 10A).

1) Enhanced Beam Centering Method

The improved beam-centering step includes two sub-steps. The first sub-step finds the beam center along the beam path by modeling the horizontal image intensity profiles (I) at each depth of the substrate using the model intensity profile (I_(M)) given by

${{I_{M}(p)} = {{ae^{{- 2}{(\frac{p - b}{c})}^{2}}} + {dp} + f}},$ where the first term models the beam intensity profile as a Gaussian function with a, b and c being peak intensity, center and radius of a Gaussian, respectively, while the last two terms model background intensity profile (e.g. ambient light, camera dark counts, non-uniform camera response) with d and ƒ being a slope and constant level of background intensity, respectively. p is the value of horizontal pixel in the image. The beam center is then found as the center of the Gaussian beam from the best fit of I_(M) to I.

FIG. 30A is a plot of the intensity I(p) versus the pixel number p(x) for a select depth into the CS substrate 10 along the beam path in the x-direction. The fit estimates the center of one of the scattered LSP light beams 216S (i.e., one of the line images in the X-shaped LSP image 248; see FIG. 4D or FIG. 30C, the latter being introduced below), as indicated by the large black dot denoted CNTR. Once a list of peak centers at each depth along the beam path has been collected, in a second sub-step a line fit is applied through all the estimated peak centers, as shown in the plot of FIG. 30B, which shows the peak centers fit in terms of the pixels located along the x and y directions, which are respectively denoted p(x) and p(y). The results of the method were compared to actual beam images and showed the method located the center of the scattered beam with high accuracy.

2) Enhanced Center Point Detection

The enhance center point detection method makes use of the Gaussian fitting method described above to track the beam centers. FIG. 30C is a schematic diagram similar to FIG. 10C that shows an LSP image 248 (intensity contour) with two superimposed fit lines FL1 and FL2 running through the center of the two line-image sections, denoted 248-1 and 248-2. The intersection INT of the two fitted lines FL1 and FL2 represents the center point of LSP image 248.

3) Enhanced Entrance Point Method

A parameter of interest for the LSP image 248 is called the entrance point and is denoted as ENP in FIG. 30C. Determining the entrance point makes use of the fitted centerlines FL1 and FL2 established by the above-described Gaussian fitting method that finds the beam centers as a function of the beam intensity. The entrance point ENP is then chosen as the half-max value on the edge intensity profile along the centerline fit for the LSP image 248. FIG. 30D is a plot of the LSP image intensity I(p) vs. pixel position p along the fitted line FL2 and in the vicinity of the entrance point ENP (see FIG. 30C). The plot of FIG. 30D shows an example edge intensity profile and identifies the maximum intensity I_(MAX), the minimum intensity I_(MIN) that represents a background or a zero intensity value (e.g., by shifting the intensity curve), and the half-maximum intensity I_(1/2), which resides midway between I_(MAX) and I_(MIN) and defines the position (pixel location) of the entrance point ENP, which in the example plot is about at pixel 32. The pixel 32 can be correlated with a physical position on the CS substrate 10 to obtain the entrance point ENP in the coordinate system of the CS substrate.

It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto. 

What is claimed is:
 1. An evanescent prism coupling spectroscopy (EPCS) system for characterizing stress in a chemically strengthened (CS) substrate having a surface and a near-surface waveguide, comprising: a) an EPCS light source system comprising: i) an EPCS light source that emits a multiwavelength EPCS light beam; ii) an optical filter assembly configured to sequentially filter the multiwavelength EPCS light beam to form a sequence of filtered EPCS light beams having different wavelengths; iii) a light guide assembly that transfers the sequence of filtered EPCS light beams as guided light to a focusing optical system arranged to receive the transferred filtered EPCS light beams and form therefrom a sequence of filtered and focused EPCS light beams; b) an EPCS coupling prism that forms an EPCS coupling surface with the surface of the CS substrate and that receives and couples the sequence of filtered and focused EPCS light beams into and out of the near-surface waveguide at the EPCS coupling surface to form a sequence of filtered and reflected EPCS light beams that respectively comprise mode spectra of the near-surface waveguide for the corresponding filtered and reflected EPCS light beam; and c) an EPCS detector system comprising: i) a switchable polarization filter operably connected to a polarization controller to sequentially perform transverse magnetic (TM) and transverse electric (TE) polarization filtering of the sequence of filtered and reflected EPCS light beams to form TM and TE filtered and reflected EPCS light beams respectively comprising TM and TE mode spectra of the near-surface waveguide; and ii) an EPCS digital detector configured to sequentially detect the sequence of TM and TE filtered and reflected EPCS light beams to sequentially capture TM and TE images of the respective TM and TE mode spectra of the near-surface waveguide at the different filter wavelengths.
 2. The EPCS system according to claim 1, wherein the optical filter assembly comprises an optical filter wheel.
 3. The EPCS system according to claim 1, wherein the light guide assembly comprises a light guide with an output end, the EPCS detector system comprises an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil by the focusing optical system.
 4. A hybrid system for characterizing stress in a chemically strengthened (CS) substrate having the surface and the near-surface waveguide, comprising: the EPCS system according to claim 1; a scattered light polarimetry (LSP) sub-system comprising a LSP light source system, an optical compensator and a LSP detector system in optical communication with the optical compensator through an LSP coupling prism having a LSP coupling surface; and a coupling prism assembly comprising a prism support frame configured to operably support the EPCS and LSP coupling prisms so that the EPCS and LSP coupling surfaces resides substantially in a common plane; and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture, and to operably support the coupling prism assembly at the measurement aperture so that the EPCS and LSP coupling surfaces reside substantially at the measurement plane.
 5. The hybrid system of claim 4, wherein: a) the LSP light source system comprises, in order along a first system axis: i) a laser diode that emits a LSP light beam having at least 1 microwatt of power and centered on a wavelength of 405 nanometers; ii) a shutter system arranged to periodically block the LSP light beam; iii) a rotatable half-wave plate; iv) a first fixed polarizer; v) a first focusing lens; vi) a light diffuser; and vii) a second focusing lens; b) the optical compensator arranged downstream of the LSP light source and configured to impart to the LSP light beam a time-varying polarization, the optical compensator comprising in order along the first system axis: i) a polarizing beam splitter arranged to receive the LSP light beam from the LSP light source and transmit a first portion of the LSP light beam along the first system axis and to direct a second portion of the LSP light beam along a spectrometer axis; ii) a spectrometer arranged along the spectrometer axis and arranged to receive and spectrally process the second portion of the LSP light beam; iii) a second fixed polarizer; iv) a variable polarizer that imparts the time-varying polarization to the LSP light beam to form a time-varying polarized LSP light beam; c) an axially movable focusing lens arranged downstream of the optical compensator and configured to receive and focus the time-varying polarized LSP light beam to form a focused time-varying polarized LSP light beam; d) the LSP coupling prism interfaced with surface of the CS substrate to form a LSP coupling interface, the focused time-varying polarized LSP light beam is focused at the LSP coupling interface to generate scattered light from stress-induced features within the body of the CS substrate; e) the LSP detector system arranged downstream of the LSP coupling prism and arranged to receive the scattered light, the LSP detector system comprising: i) a LSP digital detector; and ii) a collection optical system that collects and directs the scattered light to the LSP digital detector to form an LSP image at the LSP digital detector.
 6. The hybrid system of claim 5, wherein the variable polarizer comprises a liquid crystal variable retarder (LCVR) operably connected to a polarization controller, and the LCVR is in operable communication with a temperature controller configured to maintain the LCVR within a select temperature range.
 7. The hybrid system of claim 5, wherein the LSP sub-system is configured to perform the following steps: estimating a beam center (CNTR) of the time-varying LSP polarized light beam, where the time-varying LSP polarized light beam follows a beam path through the body of the CS substrate, by: i) performing a tilted Gaussian fit to the scattered light for select depths into the body of the CS substrate along the beam path to define a first set of beam centers; and ii) fitting a first line through the first set of beam centers to define a first fitted line; b) estimating a center point of the time-varying LSP polarized light beam by fitting a second line through a second set of beam centers to define a second fitted line, and identifying the center point where the first and second fitted lines cross; and c) identifying an edge intensity profile of the LSP image along one of the first and second fitted lines where the edge intensity profile transitions from a maximum value I_(MAX) to a minimum value I_(MIN) representative of a background intensity value; and determining a half-maximum intensity value I_(1/2) midway between the maximum and minimum intensity values I_(MAX) and I_(MIN) and defining an entrance point to be at the half-maximum intensity value I_(1/2).
 8. The hybrid system of claim 5, wherein the EPCS digital detector comprises at least first and second EPCS digital detectors, the at least first and second digital detectors configured to respectively detect the TM and TE filtered and reflected EPCS light beams.
 9. The hybrid system of claim 8, wherein the at least first and second EPCS digital detectors reside along respective at least first and second detector axes, and the at least one switchable polarization filter comprises at least first and second switchable polarization filters respectively arranged along the at least first and second detector axes and upstream of the corresponding one of the at least first and second EPCS digital detectors.
 10. The hybrid system of claim 4, wherein the CS substrate comprises a glass material, a glass-ceramic material, or a crystalline material, and wherein the near-surface waveguide of the CS substrate is defined by a near-surface spike region and a deep region.
 11. The hybrid system of claim 4, wherein the light guide assembly comprises a light guide with an output end, the EPCS detector system comprises an entrance pupil, and wherein the output end of the light guide is imaged onto the entrance pupil.
 12. The hybrid system of claim 4, wherein the filtered EPCS light beams having different wavelengths include a filtered EPCS light beam centered on a wavelength of 780 nm.
 13. The hybrid system of claim 4, wherein the filtered EPCS light beams having different wavelengths include beams centered on wavelengths of centered on a wavelengths of 450 nm, 510 nm, and 640 nm. 